I'm not sure I fully understand how XMR guarantees fungibility of its token, but I think a big part of it is the construction of ring signatures to prove transactions are zero sum, with a pool of participating signers that all have statistically valid plausible deniability. Start here maybe https://web.getmonero.org/resources/moneropedia/clsag.html?

I fully understand how privacy coins like XMT audit their supply. They assume that Pedersen commitments r*G+v*H can only be openend in one way. Which is false if the discrete log of H w.r.t. G is known. I said "I think" because there might by privacy coins not based on Pedersen commitments that I'm just not aware of.

Sure enough https://www.getmonero.org/resources/moneropedia/pedersen-com...

Whelp, I'm gonna be bullish on some kind of move from cryptocurrency to a cypherpunk / plan9-esque Renaissance of distributed social computing, maybe taking ideas from e lang, and sprinkling in some web of trust auditable ledger if its appropriate and get the heck out of (web3) Dodge.