Backtracking a bit:
I’m fairly sure that the defining element of sinas chinom is actually hating somebody; hence the term sinas chinom. Seeing as we have no evidence that any Rebbe bears R’ Lau any personal animosity, I don’t see how the term sinas chinom has any bearing here.
Also, just curious: If the Satmar/Vishnitzer Rebbes refusing to go to the Siyum Hashas if R’ Lau speaks constitutes sinas chinom, why isn’t publicly condemning their decision to do so also sinas chinom?