On the historical statement, there is simply no evidence that there were "a lot" of antitrinitarians amongst Calvinists. Some Calvinists converted to Socinianism, just as there were converts to Socinianism from Lutheranism. There is no "reason" to think that these conversions were owing to internal structures within the doctrinal beliefs held by the converts. It is just as possible that Arminianism or some other intermediate step was involved. If that is the case then the Lutheran affinity with the Arminian doctrine of universal grace might provide a better explanation for the conversion and true Calvinism might be shown to preserve its adherents from the errors of Socinianism.
On the theological question, the Trinity is understood by means of reason so as to avoid contradiction. One substance; three persons. The same applies to Christology: two natures; one person. Classifying and distributing are rational processes.
As for the gentleman's understanding of Calvinism, it is flawed. He uses the word "equal" when speaking of predestination and reprobation. "Equality" only applies to the act of decreeing, not to the things decreed. He has applied equal ultimacy to the things decreed -- predestination and reprobation. Calvinism does not teach this. Moreover, Calvinists teach the doctrine of reprobation on the basis of what God has revealed in holy Scripture. The prophet Isaiah's stone of stumbling and the apostle Paul's vessels of wrath are biblical teachings. Calvinism merely uses reason to categorise and distribute this teaching within the system of theology, noting its inherent connections within special revelation itself. Holy Scripture requires this use of reason when it commands us to "prove" all things and "understand" the will of the Lord.