I think we need to be cognizant of another recent thread when considering this request: Chromium progressive web apps. Allowing for the creation of desktop launchers does allow for sandbox escapes. On the surface, there are many parallels between these cases (it was granted for Chromium).
Looking at the .desktop files that are generated, it appears as though we see:
Exec=steam steam://rungameid/<game id>
which suggests to me that games are re-executed under the confinement provided by the steam snap, although a process listing reveals the situation is more complex than that (and I have not dug into it).
Given use case, the publisher and the profile of Steam, I am supportive, but in the name of completeness are you just able to briefly explain the (sandboxed or not) environment that steam apps run in?