Steam for windows allows you to launch non-steam games (or applications) from Steam by creating a shortcut(iirc you even get the steam overlay when you press shift+tab). I would be surprised if they removed this functionality from SteamOS.
I don't believe that's the point she is making. Obviously the distributor should be compensated. I believe her point is will 3rd parties that are NOT green lit on steam still be downloadable similar to Android's install from unknown sources.
My initial thought would be they allow 3rd party / not affiliated with steam. They seem pretty open with everything about SteamOS and I would think it would only hurt their chances of adoption if they were to lock it down. Even if they do lock that down I'm sure someone would easily find a work around as it is Linux after all.
[1] http://steamcommunity.com/groups/steamuniverse/discussions/1...