Go to the snap site and try to find a security section that describes how snap packages are signed. You won't be able to find it because it doesn't exist, and they don't highlight their own security vulnerabilities.
What I can cite is how this should work, for example how apt signs all packages by default
Note how in the above doc there's a message
WARNING: The following packages cannot be authenticated!
...
Install these packages without verification [y/N]?
That doesn't exist in snap because snap does not authenticate downloads. It'll just happily install something maliciously modified.