I did not read the Gantin statement thoroughly. I mistook it for his earlier one of JUNE 17, 1988, which was a warning. But even if it declared a latae sententiae excommunication had occurred, this is merely ex post facto. The statement itself says something has already transpired. It is not Gantin imposing anything officially, nor could he. He is doing what the Pope did with the motu proprio--commenting on an event.
ultima, it was an official judicial declaration. Look at the Encyclopedia article you cited before:
Here, of course, it is not a question of excommunication latæ sententiæ and in foro interno, but only of one imposed or declared by judicial sentence.
The difference is obvious - a latae sententiae excommunication is not imposed, but rather declared to have occured. And indeed:
Having taken account of all the juridical effects, I declare that the above-mentioned Monsignor Marcel Lefebvre, and Bernard Pellay, Bernard Tissier de Mallerais, Richard Williamson and Alfonso de Galarreta have incurred ipso facto excommunication latae sententiae reserved to the Apostolic See.
The excommunication was declared and therefore it is binding in the eyes of the Church - and must be obeyed, even if incorrect.