Ok, that's apparently a special case, that's an Intercity train (as can be seen from the "Operator: DB Fernverkehr AG" on the page above) that can usually also be used with regional tickets, but DB apparently decided that doesn't apply to the 9 € ticket. The trains are painted in DB's "long distance" livery, but I'm not sure how many passengers would interpret that as a clue that the 9 € ticket may not be valid...

Paywalled article in German: https://www.nwzonline.de/plus-wirtschaft/9-euro-ticket-gilt-...