I think I’ll have to disagree with you because underwear does make a big difference for other clothes, so why wouldn’t it help with bike shorts?

You’re going to have some particles from the anus and urethra that leak out while you ride, making your butt smell like ass. They can either be absorbed by the (relatively cheaper and easier to wash) underwear, or the expensive padded bike shorts. The shorts will still get sweaty and get washed after every ride, but they’ll have less skin and body fluid particles in them.

I get that most people can’t wear underwear underneath because of the added friction. Still stands to reason that if this wasn’t an issue, the underwear would help protect the padded bike shorts.