Yes, it was.
Having taken account of all the juridical effects, I declare that the above-mentioned Archbishop Marcel Lefebvre, and Bernard Fellay, Bernard Tissier de Mallerais, Richard Williamson and Alfonso de Galarreta have incurred ipso facto excommunication latae sententiae reserved to the Apostolic See.
And what does the Catholic Encyclopedia say?
Excommunication is said to be unjust when, though valid, it is wrongfully applied to a person really innocent but believed to be guilty. 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.
Recall the difference: "imposed" is ferendae sententiæ, "declared" is latae sententiæ. So, let us say that Cardinal Gantin declared by judicial sentence an unjust excommunication. What are the effects?
But a case of unjust excommunication brings out in a much more general way the possibility of conflict between the forum internum and the forum externum, between legal justice and the real facts ... Innocent III ... concludes that the chain by which the sinner is bound in the sight of God is loosed by remission of the fault committed, whereas that which binds him in the sight of the Church is severed only by removal of the sentence ... while seeking to prove his innocence, the censured person is meanwhile bound to obey legitimate authority and to behave as one under the ban of excommunication, until he is rehabilitated or absolved.
Msgr. Lefebvre and his four bishops are bound in the sight of the Church, per the Declaration and "Ecclesia Dei". Period.
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.