I watched Johnson's mechanics on the link and qualitatively compared them to modern Major Leaguers I've tested in my biomechanics lab. As Andy pointed out, Johnson as a true sidearm pitcher. What we've found in our biomechanical research is that successful pitchers have the following sequence:
1. pelvis rotates to face the batter (that is, the belly button is facing home)
2. upper trunk rotates to face the batter while arm cocks back
3. front knee straightens
4. trunk bends forward
Typically for successful pitchers, the back leg follows through with the foot sweeping outside the body's frame. For overhead pitchers, the foot sweeps out high off the ground. For sidearm pitchers, the foot sweeps much closer to the ground.
Instead of sweeping his right foot out, Walter Johnson pretty much kept his right foot directly behind his right knee. This was not a big deal from a safety or velocity point of view, because The Big Train followed the 4 kinetic chain steps outlined above. I predict a long, successful career for this pitcher.