I would say that seriously, so there you go, theres two.

It's a turn of phrase. The belief isn't being called unserious. The holders of the belief are. It's the "white collar speak" approved way of saying those people are dumb or otherwise not worthy of consideration.

"I don't know anyone who seriously thinks that stone applied to fibrous asphalt is not a fine roofing material"

"I do not know anyone who seriously thinks that 4000kcal/day is healthy in normal circumstances"

"I don't know anyone who seriously thinks that women are incapable of working outside the home"

"I do not know anyone who seriously thinks a bright red suit is appropriate for a funeral"

And on and on and on.

But we both already knew that. So if you're gonna be obtuse and not understand it I'm gonna be obtuse and explain it.