The 1917 Code of Canon Law has the following canons:
Canon 1258 It is illicit for the faithful to assist at or participate in any way in non-Catholic religious functions.
Canon 2316 One who spontaneously and with full knowledge helps in any way in the propagation of heresy, or who co-operates in divinis with heretics contrary to the provision of canon 1258, is suspected of heresy.
I do not know what delicts incurred automatic excommunication in the 1917 code. But, I wouldn’t think merely being “suspected of heresy” as noted in canon 2316 in reference to canon 1258 would suffice.
Bill Donohoe is not a canon lawyer. He is zealous in his defense of the Catholic faith, but I think it’s a stretch to say Hitler was excommunicated latae sententiae for participating in the non-Catholic wedding.
I’m sure Hitler was latae sententiae excommunicated somewhere along the way by his actions and inactions-- but not necessarily for that.