Are you launching the game from Big Picture Mode, and is the Steam controller set to a controller profile? At least when I tested here in Windows, it was still recognized as a controller by the game with the overlay disabled, and I could load a game, open the save window and enter text without the overlay keyboard. There was a warning on the game page about the overlay being disabled and some functions unavailable, but I didn't notice any issues in the short time I tested.
If you hold Cmd when starting the game in BPM and check in the Controller tab, is the Steam controller listed there?
