Joe Biden wins primary election in New Hampshire despite not even being on the ballot

Essentially, the DNC decided to shake up the primary season by shuffling the traditional order of primaries, and had South Carolina as the first primary in February. New Hampshire has a state law requiring them to be the first primary, so they could either break state law or defy the DNC. (Keep in mind that parties are technically private organizations, so I don't know how state law can force them to do that, but American elections are weird, and I'm Canadian.) The Republican-controlled NH government decided on defying the DNC, so although they technically held a primary, it was not sanctioned or authorized, and thus the DNC disallows contenders from appearing on the ballot, which Biden complied with. I believe the DNC has also invalidated the electors as a result, so they might not even count them at the convention.

TDLR; DNC changes primary schedule, Republican NH says "by law we go first", DNC declares NH primary unsanctioned and tells candidates to stay off the ballot.

The DNC is not holding a primary so NH is just doing this on their own for fun, and to comply with it's own laws. Because there is no contest, there is no need to collect the signatures and hire the lawyers and volunteers to get into the ballot. Other candidates did, but the national democratic nominating convention will be uncontested and Joe Biden will be the nominee.