All of which is entirely irrelevant to a passenger purchasing a reservation for a DB train on the DB website. If SNCF have a problem with that then it is between them and DB, the passenger has used a valid ticket with a valid reservation, end of story.
I wouldn't want to rely on that interpretation.
If the rules say an Interrail pass requires a quota-controlled supplement, it's not clear to me that getting a much cheaper reservation for that journey out of DB would make it valid.