I don't get it. if Biden isn't on the ballot yet, and it's just a Dem placeholder until the convention, then surely it would make no difference? or alternately they will try to kick off the Dem no matter who it is?
It only works after he gets on the ballot but Heritage wants to sow doubt so that Biden is the one on the ballot, because Heritage correctly understands that replacing him hurts Trump's chances
the point is less about actually succeeding at keeping a successor off the ballot and more about creating a permission structure to call the entire election a fraud.
(they'll do that anyway, but they like to have their little legalistic hooks)
Heritage thinks they can pursue a legal strategy to keep [not Biden] off the ballot. The analyses I've seen suggests that would be unsuccessful and that the major parties have broad latitude over ballot access under all state law. So "Heritage will mess with [not Biden]" is speculative, but possible
It looks like most of their plans are about what happens if he's removed *after* the convention, because you're right, ballot access is pretty cut and dry if he just is never the official nominee to begin with.