They seem to be doing okay:
Availability list | Denim Air
Also offering Q300s and Fokker 100s.
At the moment the whole thing is more or less a paper tiger, so if they get off the ground, Denim would have time to ramp up capacity. The guy behind JOIN has been involved in literally any Dutch regional carrier of the past 25 years (BASE, Fairline, TTA, VLM), so I guess he also has close links with Denim.