While it would have been better, in my opinion, to serve the notice, this is a relatively toothless rule. A party’s failure to file and serve a notice of a constitutional challenge to a state statute does not result in a forfeiture by any party of any constitutional claim that the party has timely asserted in the action. Fed. R. Civ. P. 5.1(d)
It might be possible that the court would sanction the responsible attorney under § 1927, however at this point in the appeal that seems pretty unlikely, especially considering the lower court also had an obligation to notify, and either failed to do so, or if it did then it is a somewhat moot issue. Worst case scenario here is remand back to the lower court to give the AG a chance to intervene. But considering that the the AG did receive notification in Richards and did not intervene, remand seems unlikely.
So I'm not sure the development is terribly important.