You are 100% correct. But he is usually lumped in with Democrats in the public eye, regardless of official party affiliation, and AFAIK he is the most prominent supporter who hasn’t retracted the endorsement yet.
I’m also not implying that Sanders’ silence gives Platner any kind of claim to legitimacy, but I feel like if Sanders does come out against him, it’s the final punctuation mark.
He’s done anyway. This is a political avalanche.