I mean, it’s absolutely a sign of weakness - which is to say, it’s a sign that the incumbent isn’t popular.

It doesn’t have to be, though. Even framing it this way is kind of playing into the DNC’s hand on this matter. A primary just means that other people think they could do a better job of it than the incumbent, for whatever reason. It could be that the incumbent is unpopular, but it could also be that the challenger brings a new perspective or new knowledge to the table that makes them more suitable to hold the office. It could just be someone who wasn’t eligible to run in the previous election for that position, but they are now.