So I spelunked into the directory tree of the Ubuntu repos and it seems like debs that alias to snaps have "snap" in their version, e.g.:
firefox_1snap1-0ubuntu2_amd64.deb
and the steam package from multiverse doesn't:
steam_1.0.0.45-1ubuntu1.2_i386.deb
But it's probably a better idea to use the deb that Valve provides directly. They have a [PPA](https://repo.steampowered.com/steam ) for it that you can add so you don't have to manually fiddle with deb files.