Stallman is being consistent with his previous statements yet surprises me by being quite pragmatic about the issue.
He has a valid point. You have more freedom if you run proprietary software on free OS than when you run proprietary software on proprietary OS. As a bonus point presence of Steam on Linux will probably bring fixes to free software graphics and sound components.
Why not link to Stallman's actual post? https://www.gnu.org/philosophy/nonfree-games.en.html