Are you sure snapcraft requires the original developer publish snaps? This seems unlikely, but they may have updated their policies.
Edit: they aren’t, Signal for example is an unofficial snap not published by the Signal developers but rather “snapcrafters” - snapcraft.io/signal-desktop. This is very similar to how Flathub handles unofficial packages, except Flathub seems to have more gatekeeping (Snapcrafters doesn’t allow just anyone, but you don’t have to be part of that group to publish).
Snapcraft has hosted multiple malicious applications, so I wouldn’t exactly call it a safe place either.