Specifically his use, and type, of diffusers at early reflection points, his treatment of the front wall and putting thin, flush-mounted absorption in the corners rather than bass traps. Also he's put diffusers in the rear corners and just one small curved diffuser behind the listening position.
I liked his visual acoustics video but this seems a bit misleading.
Poly "deflectors" make sense in large rooms like in that photo, and they're at least better than a flat wall. Everest shows them used in large recording studios too. But in a bedroom size space they are totally inappropriate IMO. And even in a larger room a real QRD is better.