They aren’t required to fund him, that’s true, but all campaign donations from the DNC are transparent. He can compare his support to Adams’ 2021 purse. They also can’t stop him from fundraising directly.
If he makes it clear that they are withholding financial support given to prior candidates, then people will donate to him directly, and the DNC will be publicly called out for favoring past candidates. That’s the last thing they need after the Debbie Wasserman-Shultz/Hillary Clinton scandal.