Wonder if it's a lightweight concrete panel?
Probably costs a bit, of course, but compared to building and plastering a similar wall, it's surely worth it.
Would have worked perfectly for me. Good to know. Now I have something concrete to go on. Also good that you don't need an air gap against the existing combustible wall behind. That saves valuable space. Which, of course, also does the only 5 cm distance to the stove. All in all, a brilliant product it seems.

Now there's a real and practically functional alternative to suggest to Mr. chimney technician.
Great forum this one.

You often manage to find alternative solutions when you're stuck in your little chamber.