As far as the ACAA goes, remember that there's no private right of action: the only means of obtaining redress is a complaint to the DOT.

DL and the other airlines making the rules stricter are probably doing so in the expectation that when any complaints reach the DOT for a ruling, the DOT will essentially ignore inconvenient parts of the ACAA and rule that passage can be denied on the basis of not wearing a mask.

ETA​: SCOTUS may eventually revisit current deference to the regulatory bodies when it comes to interpretation, and DOT making up a new rule from whole cloth like this would be a pretty good case there. But a SCOTUS ruling on this is far enough in the future that the eventual damages won't matter.