From what I can see the bug is about packages, not snaps Package is multiple files app needs replaced, so you have old app getting the files replaced under it

From what I can see the bug is about packages, not snaps Package is multiple files app needs replaced, so you have old app getting the files replaced under it during upgrade, that's why restart is needed. In case of snap there is layer between, the "snap" is just a blob that is mounted by a daemon that then runs app off it. If you just move/delete old snap the daemon should just be accessing old one till the app stops