Part of interpretation that seeks a fair understanding of authorial intent is to ask with sympathy what possible meanings the writer could have conceived, and from those to choose what seems his most likely idea, founded on the context of the statement in view.
I would argue, in re. Col.1:5-6, that Paul does not reasonably think that Christ's ministers and message have gone successfully (as of yet) even to every corner of the Roman Empire, let alone the rest of the world. Paul wrote in Rom.15:24 that he hoped, with the help of believers in Rome, to get to Spain to preach the gospel, quite possibly where it had never yet come as he preferred not to "build on another's foundation," v20. Therefore, it seems sensible to try to think of alternate import of Paul's words.
His words, "all the world," is probably more relative than absolute. We could gloss the meaning as, "everywhere." When we use that term in reference to ourselves, we never mean "everywhere" in the same sense as God's omnipresence, i.e. "filling all space." No, but we most often mean it colloquially, relatively, and in connection with whatever is the subject of our thought in the moment. If I say, "I hear that song everywhere," I don't mean that it is constantly in my ears, without a break, every waking instant--I might be (or go) insane, if that were the case literally.
Paul hadn't even finished preaching the gospel over the course of his life around the Roman world, and may have finally got to Spain after he was released from arrest (in Rome, whence he wrote Colossians). Yet, to his reckoning it was still possible for him to assert most sincerely that the gospel had gone forth into all the world, and in particular had not somehow bypassed the smallish Asian city of Colosse. It was no longer "confined" as it were to Jerusalem, Judea, and Samaria; but had started to spread out, like water seeking its own level, finding ways of least resistance, eventually bypassing barriers. It was only a matter of time until all the land was well watered.
We might call Paul's view here: the mid-range perspective. If he could only see a local reality, he could hardly claim a universal fact based on limited observation. If he envisioned things globally, he might qualify his (apparent) world-wide claim with a "so far;" but even then he might otherwise think in terms of momentum and the lack of effective opposition to the eventual flood. The mid-range perspective of "the world" positions Paul where he can see back to the Jerusalem source, with its wineskin burst; and his trail of missionary labor, and the paths of others to the left and right; and all around him in Rome in what is the political and commercial hub of his "world," where even in his personal confinement he knows Christ's influence is already being carried even further by the momentum that had not stopped because he wore chains.
Still, one may object to all that commentary by pointing to Paul's language in Col.1:23, "every creature under heaven." It's a fair point; but again to understand what Paul means we need to remember things we already determined, and to make reasonable inferences as to his intent based on the context--literary, personal, cultural, etc. We can instantly reduce the maximal universalism of the words from any meaning that includes all the beasts, birds, fish and insects (if we've already excluded the mineral and vegetable and microscopic world without even thinking). Paul does not mean that every molecule of existence and aggregate object has received an individual, intelligible articulation of the gospel hope.
Nor is it reasonable to think Paul has now supposed (since he wrote his Romans letter) that others have fulfilled his mission in spite of his incarceration. We could chalk up his expression to exuberant hyperbole; i.e. that being so enthusiastic about the success of Christ's power, Paul exaggerates the present reach of the missionary enterprise (while expecting it shall be done). I think there's a better understanding. I think Paul is reflecting on the power of the resurrection, Christ's ascension, and the outpouring of the Spirit. In principle the gospel has been announced into the heavens and throughout the earth; and though it is not effectual for salvation into hell, the echoes of Christ's victory have been heard there also, to the eternal chagrin of the dark domain.
Pentecost reflects this proclamation by the manifestation of overcome language barriers. In one location, despite the variety of tongues the conditions were ideal for a gospel message propagated without hindrance. The time for calling the Gentiles was now, see Is.34:1, "Draw near, O nations, to hear, and give attention, O peoples! Let the earth hear, and all that fills it; the world, and all that comes from it." This chapter contains references to both men and living creatures that hear, which is followed by (ignoring the ch. division) salvation. It is even more demonstrable later in Is.45:22, "Turn to me and be saved, all the ends of the earth! For I am God, and there is no other." These words were preceded by terms such as, "that people may know, from the rising of the sun and from the west, that there is none besides me," v6; and "the Lord, who created the heavens (he is God!), who formed the earth and made it (he established it; he did not create it empty, he formed it to be inhabited," v18.
Is.66:19, in the final ch. the Lord declares, "and I will set a sign among them. And from them I will send survivors to the nations, to Tarshish, Pul, and Lud, who draw the bow, to Tubal and Javan, to the coastlands far away, that have not heard my fame or seen my glory. And they shall declare my glory among the nations." Paul sees the fulfillment of this prophecy has in fact come about, if not in finished form, yet in reality. Ps.67:4, "Let the nations be glad and sing for joy, for you judge the peoples with equity and guide the nations upon earth. Selah" What I think is that Paul puts himself and the Colossians into the frame of the picture of this fulfillment. as if to say, "You are part of this prophecy, that is right now in the process of completion." If it takes another 2000yrs to wrap up, such delay only bears witness to the sheer volume of space-time necessary to compel an adequate hearing by "every creature under heaven."
Paul's statement was justified even as he wrote it. It has only become moreso in the time intervening until now. For the brethren then, at Colosse where Paul in person had not even passed through, they were encouraged that the same gospel that had been proclaimed in so many other places had also been delivered to them.
