As a formal matter, it's true the delegates would be unbound and Harris wouldn't automatically be the nominee. But these pie in the sky schemes for some American Idol competition are nonsense, because no matter what the formal process is, nobody who matters will challenge Harris for the nomination.
Every single one of these fantasy football picks--Newsom, Whitmer, etc.--would endorse Harris the day Biden drops out. And then it doesn't matter what over-engineered 'mini-primary' scheme you come up with, she's running effectively unopposed and is thus the presumptive nominee.
Wow, you're right. This is one of those smart ideas where once you hear it, it's so obviously true, that you can't believe you didn't realize it before.