As to fact: I supported Theotokos (Th) LONG, decades long, before I supported Immaculate Conception
(IC). The Episcopal Church (back when it mattered) was silent but leaning no on IC but supported Th. My argument for Th did not change in any way because I assented to IC. I do not mention IC in my defense of Th because I do not see any relevance.
As to the logic: it seems to me that the conclusion of Th follows from the reasons set forth in the Chalcedonian definition which, as far as I know, does not contemplate coming down one way or another on IC.
It seems to me that in the background of his criticism of your logic, there lurks an unstated premise/conclusion (not sure which) that Jesus could be God ONLY if Mary were immaculately conceived, which I think would be absurd.
Am I reading this correctly? The disclaimer here is that I do not possess a Ph.D. in Philosophy or Logic.