yeah he does have a platform

But no meaningful platform. He doesn't care about it, his supporters don't care about it, the odds of him passing anything significant are slim, etc. It's just "hey remember Obama?"

it’s an opportunity to make clear that you don’t support something

You can't make anything clear with your vote, because a vote isn't a message, and the left doesn't control mainstream messaging outlets. Literally zero of the reasons you have for voting the way you do will be communicated to anyone unless you personally tell them. The messaging is all done after the fact, by people in the media.