That may be true but the sense I get from Gen Z folks who I've talked to is that for them Discord is almost more of a social network for interacting with a community. Their activity might be centered around gaming but they're using Discord to find people they don't already know to play with, talk about the game, etc. For those folks, Mumble will not be even close to an alternative to discord.

Myself, though, I basically only use it to talk to the same guys I've been gaming with since we met in middle school 20-some years ago, and for that Mumble seems perfect.