I don’t think you do need Steam running. If it’s truly DRM-free, just copy the game directory to a new machine and the game will run. Don’t launch through Steam, launch it directly from the game directory.
I’ve run games directly without Steam running on a handful of occasions, such as when someone else is using my Steam account (e.g. my kids on my other computer) and I want to play a game. I could probably play in offline mode I guess, but running it directly isn’t that hard.
It’s not an installer, but I don’t need an installer when I already have all the game files in one directory.