Most Intercité trains in France have mandatory reservations, including this one.

Support has replied to me:

“I am sorry for all the confusion that has been casued by this matter. Hopefully I can clear it up.
 
There are some trains travelling this route that do not require reservations, but others, like the IC4659 that have mandatory reservations. Different train has different amenities and travel at different speeds, more direct routes etc. There a a lot of reasons why one train may require a reservation and why some may not.
 
Everything what I can see form my side says that reservations for this particular train are mandatory. As for the people that have been travelling without a reservation, they may be just regular commuters, or have bought a train ticket. Keep in mind that often the reservations as you understand them only apply to pass holders.
 
When in doubt regarding reservations, always defer to the information found in our online timetable, this is the most current and up to date.”

(Another) support agent also told me the responsible (i guess app) team will be noticed.

Actually also thanks to that amazing support! Interrail support replied me within less than an hour via mail. I did not expect such a great support experience!