The use of high temp composites/paints has been used for engine fire protection for years on a number of aircraft. But the intent of those firewalls and “curtains” is to only prevent the fire spread for a specific time and not fully contain it. However, even on the certified side any configuration, metal or composite, has its limitations has seen in the FL EMS EC135 inflight fire in 2023.
Regardless, as to the effects of the exhaust gases on the airframe, with the use of exhaust ejectors and deflectors I don’t see an issue with developing a suitable solution to protect the airframe. And its good to see he still is following the certification guidance which is one of principal reasons a number of people have bought production slots.