Perhaps His Excellency Bishop Fellay is thinking only of saving the body of the SSPX from going down on His Excellency Bishop Williamson's sinking ship.
I love and revere both of these holy bishops, but I can see the prudence behind Bishop Fellay not wanting to kill his missionary work in Rome (and I say truthfully "missionary work," not "getting together") over a question of history. The questions of the Faith are of primary importance, and NEED to be returned to the minds of the churchmen in Rome. Questions of history are not necessary to the soul, and however His Excellency Bishop Williamson's position stands in relation to the truth, taking the enemy's bait and having the whole effort to right Rome again be shot down on a political point, I think, would not be very much in line with the duties they have, none of which is to make sure no one errs in matters of history. Though I'm sure the devil would like it ridiculously. (Kind of like the great joke of getting pious Catholics to disobey God in the name of obedience... Getting Catholics to abandon or loose the fight for the True Faith in the name of truth? Doesn't that just smack of the devil's irony!)
Yes, it is very, very unfortunate that the world has lost such an able mind in the fight for the Faith on the main lines... but Bishop Williamson will be lucky if he doesn't do time in a prison for what he said (true or not), and given the whole world's hysteria on the topic, I can't imagine by any means that attacking on this point would serve in any way, shape, or form to do aught but shoot down the whole fight for the True Faith in Rome. And over something no one "educated" in a public school would probably ever reconsider no matter what anyone said.
Yeah, if you're a single rebel out there, living dangerously on your own, then you can spout whatever you want, and the consequences effect only yourself. But when you're a bishop in charge of souls, and one of only four among probably not many others fighting for the True Faith, and when you have a whole society... one of the only societies... of traditional priests depending on you to lead them, getting yourself crucified in the media (and thereby discredited across the board so no one you need to reach would ever listen to you) by attacking an issue like that is not the most prudent thing to be doing.