Really curious about the "supposedly" in this. Does Nassim not actually believe or endorse this view?

I read it as him endorsing the empirical proposition, but not the value proposition. That's the point of the second sentence- he wouldn't say "these are good friends to lose" if there wasn't the potential to lose friends.

Taleb is responding to the common claim that one shouldn't call out BS because one will thus loose friends.

Nassim doesn't advocate that you should become totally uncompromising toward BS as a technique to change the people with whom you hang out.

He doesn't advocate that you should optimize your tactics in that way to get better friends. He rather advocates authenticity where you don't hide by avoiding to call out BS because you are afraid of offending other people.

If being authentic means calling out BS than do it. If being authentic for you means to not confront other people about the BS they talk about, don't.