Snap is far more like nix. Flatpak deals with a limited subset of what nix and snap do (e.g. it can’t distribute kernel packages).

While snap is certainly not without its problems, people repeatedly make massive negative claims about it that, while often based on a core of truth, are highly embellished to the point of being misinformation. (This is the same tactic I see with bad-faith political trolls, and with a similar result really - they’ll consistently try to use that core of truth to make far stronger claims than are defensible and, when it’s pointed out to them, they move the goalposts to a smaller, more defensible claim, only to repeat the bigger, debunked, claim later.)