There is no "but you're good enough for the grant if you're good enough for the core" or "lose the 15k" or whatever. It's not there to be an independent program, it's there so they can cover bets they otherwise lack the space to engage.

Think about it in terms of logistics.

They go over applications, which are attached to core program applications, so they decide who to accept, then they grab the pile that was good but couldn't fit and award grants to some of those that sent the auxiliary application for a grant. Then they send award letters for both.

After they've sent letters, whose grant do you propose they retroactively deny to make room for someone who got the main prize but changed their mind about accepting it and just wants money?

They could hypothetically set it up so you could pick one, but that adds a bunch of extra work to administer it, plus you'd get people trying to game the process by sending in core program apps they don't intend to follow through on. So it doesn't make any sense for them to do it that way.