Would that also not equally apply to Keyes or any other candidate on the ballot?
Problem is, there is no requirement, except in Hawaii, for candidates for the general election to certify that they meet the Constitutional requirement in order to appear on the ballot. So, in not doing that, Obama was not treated any differently than McCain or Keyes, who also were not required to certify eligibility, although IIRC, the Republicans did it anyway.
It might. Carter doesn't completely discount Keyes as a plaintiff with standing. Furthermore, you would probably find a more sympathetic ear in a state court and in a pre-election suit, where the "stakes" would be so much more dilluted - you'd just be arguing someone either should or should not be on the state ballot.
"Problem is, there is no requirement, except in Hawaii, for candidates for the general election to certify that they meet the Constitutional requirement in order to appear on the ballot."
I can't speak to HI or the other 48 states, but I can address my state OH. Ohio's revised code does require that candidates in primary and general elections for POTUS sign a "Declaration of Candidacy" (the process is outlined on the Ohio Secretary of State website). That candidacy requires that you swear, under penalty of "election falsification", that you are Constitutionally eligible for the office of President.
This is one reason I'm so dubious of the many stories that have come out recently that there was some impropriety with the DNC's certifications of eligibility. Ohio doesn't accept a standardized form. They only accept their own, propriety declaration of candidacy. I suspect Ohio isn't alone in that requirement.