The Third Circuit had ruled on a different principle that says if you make a minor mistake that’s not material to the reason for the rules, that’s not worth losing your vote over. The state courts had generally agreed. Since the purpose of the part that was being signed and dated is to say “I attest that I am who I say I am and I am qualified to vote,” the date part was not “material” because the important date was the date the ballot was received, not the date it was signed.

People were making other mistakes with the date, like putting their birthday or the date of the election there, and election boards were counting them; it was only the undated ballots that were being tossed out.

The Supreme Court did not say anything about why it was reversing the Third Circuit ruling that the ballots should be counted, except to send the case back with orders that it be declared moot. So another case could potentially hinge on the same arguments about the materiality of the date (and there were some cases in state court moving to do so in the Oz/McCormick primary until McCormick conceded).