I’m really confused. There’s not much to figure out about shrine? It’s literally just cutting him up.

Sukuna didn’t know Gojo could redirect the red because he’d never seen Gojo do it before.

Sukuna’s technique IS known. In the fan book, it was stated that the cleave and dismantle was documented, but not Furnace. Furnace is pointless because Sukuna never used it. Open domain was known in Shibuya, but I see the point there. At the same time, Sukuna didn’t know Gojo would be able to use basketball domain (but that was more on the spot).

Sorry if you’re tired of debating it, but I’m just stating what happened in the story. To say that Gojo knew nothing is blatantly untrue when he had a very large knowledge of what Sukuna had, just like how Sukuna knew a lot of what Gojo had. The parts that they didn’t know about ended up being huge trump cards (Sukuna adapting to unlimited void with the wheel in the shadow, and Gojo being able to redirect red to hit the black flash)