Best-Written Campaign Setting?

[Psyren]

Having no prior knowledge of who he is, I just assumed that Xanathar was the beholder equivalent of a "man baby" crime boss who gets anything he wants.

Maybe he is a genius and the commentary is completely off. Or he is purposely trolling everyone.

As I mentioned above, there is not just one "Xanathar." The current bearer of the title is childish because they are a young Beholder, hence the caliber of the sidebar jokes in XGtE.

Mark might have been thinking of Kirukeskai, the most famous crime boss 'Xanathar' throughout 3rd edition, or its predecessor the original Xanathar (which might have been that creature's name) who was secretly slain back in 2e.