I think you meant "Any legal challenges about citizenship are technically premature for the general election". If the question is the nomination, the challenge is not premature during the nomination process. It is a moot challenge if the unqualified candidate gathers little or no support, and it is a moot challenge when brought by the winner.
The way I see it, the best argument in both primary and general election contests looks at ballot integrity, as a function of the government (state election offices) not facilitating fraud on the public.
I would argue it's moot for the party nomination as there is zero requirement that someone be a natural born citizen to become a party's nominee. Only to become president.
If they wanted, I think a party could nominate just about anyone regardless of age, citizenship, etc. But it does not mean they would qualify to be president.