Why should you expect in general that someone who agrees with you on one issue also agrees with you on other issues? The idea that Israel is the greatest threat to free speech at this point in time is as offensive to some people as the idea that Russia is fighting a just war in Ukraine is to some other people.

Their message seems to suggest that the claim is worth more because of who it is attributed to. Without Mearsheimer's name, it's just an utterly generic observation.

The attribution would make sense if Mearsheimer had actually said something novel, or if his association somehow made the statement more credible.