That's battery not assault.
All I have to do is touch you, no matter how slight, for it to be assault.
That's why you get charged with assault for spitting on a police officer
You don't have to actually make physical contact with someone for it to be assault. If I swing a baseball bat at you, or a flyswatter, Assault.
If I hit you with either, battery.
Then you have the whole ablility and intent to harm, but that's geting into it more technically.
California Penal Code 240 - 248 :
240. An assault is an unlawful attempt, coupled with a present
ability, to commit a violent injury on the person of another.
Was it technically assault, yea. It was also battery. But no attorney would ever actually file on this, at least most of them wouldn't
If the shove had caused him to fall or hurt himself, different story. Either way the guy was out of line.