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.
That happened because California had extremely easy ballot access, so a bunch of joke candidates could file and get on the ballot. Not really applicable here. There's no 'getting on the ballot' except at the convention with some minimum threshold of delegate support, which nobody else would meet.
Under this proposal (which is just some memo this professor wrote), it'd be the top six with no minimum threshold. So, like, even one delegate potentially. But that's dumb and not how it would actually work. You set a minimum threshold, 10% of the delegates or whatever.
I don't have it offhand but I recall there's already something like this in the DNC rules. To be recognized for the roll call a candidate must have, I want to say one fifth but something like that, of the delegates co-nominating them. That's how you make sure you're not flooded with fringe nobodies.