This is so silly. If the Democratic Party has set up rules for running as a Democrat, and he’s following the rules, why the hell is it his fault if you think the rules should exclude him? He’s not the one who set those rules up.
If you don’t like how he followed the rules to run as a Democrat, direct your snark at the party officials who failed to change the rules in 2017, and who likely won’t change the rules now.
Politics isn’t a team sport. He used the mechanisms of power by the rules those mechanisms set up to try to change the balance of power. You can’t ask for fairer than that.