You are wrong. The Holy See is an ambiguous place reference--"See" meaning "Seat"--and includes those bureaucrats who work directly for the papacy. The term is used just as people might use the term "White House" to refer generally to a President's administration. If someone were to say "the White House is full of traitors," for instance, that would not necessarily be tantamount to saying the President is a traitor. And, in fact, the letter you cite shows Lefebvre referring to men in the plural as Antichrists--those who hold very high offices in Rome.
You don't want to believe this because it doesn't fit in with your scurrilous caricature of the SSPX and of Archbishop Lefebvre. But, in fact, the Archbishop made it very clear he could not cross the line into sedevacantism, nor was he willing to charge the Pope with heresy, though after Assisi, he was sorely tempted--and even actively purged from his ranks those priests of the Society who would not acknowledge the legitimacy of John Paul II. So your point is absurd--especially in light of the fact that he urges--in the same letter you quote--that his followers never abandon the See of Peter.
*Applied politically, they'd be "The White House and the Legislative Branch being occupied by anti-Christs" could ONLY mean Bush was an antiChrist.
I know you don't see that obvious meaning. Sane men do.