In practice, but that’s because the party has always defaulted to supporting the incumbent.

The sitting president can’t just declare they’re the only option.

However what decides the candidate isn’t a primary election, those are completely nonbinding and more like a survey than anything.

So even if Biden lost the primary, the DNC could say he’s the candidate anyways.

So in practice you’re right. But saying it the way you did makes it sound like it wouldn’t be affront against democracy if it came down to the president losing the primary and the party running them anyways