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...
People are used to use a regional ticket with those trains, and they show an "RE" number. This will likely lead to loads of discussions on the train if they will follow through with that.
Paywalled article in German: https://www.nwzonline.de/plus-wirtschaft/9-euro-ticket-gilt-...