No one was on base so it's not a balk, however, it is a balk-able offense so it should have been called a ball worst case

I believe it wasn't a balk because his left foot never came off the rubber, and his right foot never touched the ground.

If there were runners on base it would be a balk. But you can't have a balk without runners. The umpires didn't declare it an illegal pitch so at the time you could only really warn him for unsportsmanlike behaviour then eject if he did it again, but a day or two ago the MLB came out saying that yes, it is actually an illegal pitch and been a ball (with no runner) or balk (with runners).