Bishop Williamson also said, "You can't make anything but a butter agreement with people whose minds are made of butter!" (Or words to that effect.) Meaning... the men in Rome SAY one thing, but what they mean is something totally different from what they know the person or people in question will think it means. And after you've made an agreement... you with YOUR understanding and them with THEIRS... thinking you've made a great deal for yourself, suddenly the tables are turned, and you find out that they mean something totally objectionable by what they said, and now you've agreed to it! Which is precisely what happened, as I understand, to Archbishop Lefebvre. (Concerning a certain signature.) When he realized what they meant... how could he agree to it?
I'm sorry to say it, but a blind man could see that the men in rome have been playing mind/word games with the traditionalists all along. And reality has proved it in those traditional groups who went along and made an agreement with rome, being promised all sorts of things, and understanding the agreement in one way, only to later find that the agreement isn't at all what they thought it would be, and that very quickly they're under the thumb, so to speak, to accept modernism hook, line and sinker, and to do it silently! The SSPX observed this not many years ago, and saw what came of it.
For the SSPX to make an agreement right now, would be like watching someone go onto a used car lot, get promised a perfectly excellent deal, and end up with a lemon, and then proceed to go do the same thing yourself. You KNOW they're sharks, why on earth would you follow suit after someone else just proved all of your doubts to be facts?