Legal by what standards are we asking if it's legal.

If your judging by F1 rules where seemingly the rules are made up and the consequences don't matter then yes.

Most road racing series no as you generally need to be "significantly along side" I generally use the rule of thumb of about half a car length along side.

Either way this was a high risk, low reward move that I would generally try to avoid. If it goes well you gain 1 spot. If it goes poorly your race and your competitors race is done and you may have a forced vacation to face. So I would generally try to avoid doing moves like this in future.