I see what you're getting at, but for constitutional purposes I think the plaintiff--even with backup insurance--had a "concrete" interest insofar that he signed up for Obamacare. That he was or could be covered by the VA insurance is neither here nor there. It does beg the question: if he had VA "insurance" (is that even a thing?), then he shouldn't have signed up for Obamacare. But if he paid for Obamacare--so long as he was entitled to sign up for it--then he had a 'concrete interest' in the matter.
And even if the no plaintiffs did, I think the SCOTUS would likely ignore that and rule. It'd be a glossing over of an otherwise necessary issue, but has arguably happened before.
Then I wonder why Justice Ginsburg asked the question? The Justices already knew in the briefs that all of the plaintiffs had enrolled in the ACA.
BTW, perhaps I should have said the attorneys had answered Ginsburg in the affirmative. I meant negative in the sense of whether the plaintiffs only had an ideological interest in the ACA, which the attorneys denied. Or did I state that wrong again? Oh, my, I have such trouble with negatives.