I really wasn't trying to argue for this theory, and it may be totally ineffective. It's your area of expertise and I trust you on that. But I would still insist there is a rationale, even if it's not a good one, which is frequently espoused by leaders representing groups that care about this issue.
Threatening not to vote for Biden and thus making the election more likely of a far worse candidate--both generally and on this specific issue--is like trying to take yourself hostage.
It might work in Blazing Saddles but it doesn't work in real life.
I just don't think it makes sense to act as if they are doing it for no reason at all or simply because they are selfish. They care about an issue and are trying to enact change. They probably have the political calculus wrong, but that doesn't mean that they are irrational.