In the beginning of the majority opinion (page 46)

The Court held that “when the Executive exercises this power [to exclude an alien] on the basis of a facially legitimate and bona fide reason, the courts will neither look behind the exercise of that discretion, nor test it by balancing its justification against the [plaintiffs’] First Amendment interests.”

If that is part of the precedent it would seem that the issue brought up in that quote isn't a paradox at all since the court held that the reasons had to be bona fide (in good faith). They use a really subtle reading of Din majority to back that up

Absent an affirmative showing of bad faith on the part of the consular officer who denied Berashk a visa — which Din has not plausibly alleged with sufficient particularity — Mandel instructs us not to “look behind” the Government’s exclusion of Berashk for additional factual details beyond what its express reliance on § 1182(a)(3)(B) encompassed.

Which could be read to say that if the showing of bad faith was shown then they could "look behind" and not be in opposition to Mandel. It should be noted of course that the dissenting opinion takes issue with this reading of that line from the Din opinion and launches their attack from that point as well.

Anyway, if one sides with the majority interpretation this could be used to resolve the paradox that Dershowitz brings issue with -- even though the larger discussion about that issue is an interesting one.