Ujm’s point, i think, isn’t that rav shach was alone, but rather that userb’s theory of him being a spokesperson is off – i agree with ujm on that. Rav shach wasn’t the spokesperson for gedolei yisroel; gedolei yisroel listened to his opinions because he was the biggest of his generation.
So the other gedolim agreed with him on chabad, for the most part, because they deferred to him; yserb has the chain of command backwards.