Confused if a move is/ was legal?


No, it's not a legal move; however, you're not alone in asking that question. In fact, questions like yours are very often raised on the chess.com forums. Moving your king next to the opponent's king is placing it in check, regardless of any other circumstances. The definition of check is basically a situation where your opponent would be able to capture your king on the next move, if capturing kings were allowed. By saying that black can't capture your king because doing so would put your king in check, you're violating the rules of check yourself while not letting your opponent violate them.