The court is saying, if we had litigated this (and most likely upheld it, because Chevron) then it could stand now, even after Loper Bright. But if we never looked at it before, then we get to use the new rule.
That doesn't make a lot of sense.
Like, the counterfactual is they litigated and upheld the order, so it's safe. But because it was never challenged before, it can be challenged now. Only orders that were challenged are safe.
It would make more sense to do it the way I thought: everything, even if it wasn't challenged, that would have relied on Chevron still can rely on Chevron. That gives "the benefit of the doubt" to the agency that passed the regulation in the Chevron era.
In the new scheme, the regulators have to comb through all their orders going back decades, figure out which ones were never litigated, and figure out how to defend it without Chevron.