Phillips received 19.7% of the vote in the New Hampshire primary, despite Joe Biden not being on the ballot. Phillips received less than 2% of the vote in the South Carolina primary.

https://en.wikipedia.org/wiki/Dean_Phillips_2024_presidential_campaign

Each state sets its own primary rules, so they're all over the place. If the guy can't get enough support, he can still be a write-in.

Sorry, but that's not preventing someone from running.

Dean Phillips? I'm fairly engaged in mainstream political discussions and the only reason I've heard his name is to point out that he's one of a small number of potential spoiler candidates.

Even the article you linked is full of quotes bashing his campaign.

The guy taking campaign donations from Harlan Crow ought to be enough to tell you everything you need to know. This guy isn't here trying to win, he's just trying to muddy the waters.

I know Biden is a really tough pill to swallow. It's true, I feel it too, but unless he drops dead tomorrow, he's the guy that has already beaten Trump, so smart money is going to the proven winner. The sooner we all accept that and start focusing on the down-ballot races, the better. Because truthfully the president doesn't make enough difference if Congress continues to be absolutely fucking useless.