Custom in this case doesn’t really need to carry any weight either, it could be a simple voltage bump, clock bump, laser cutting cores etc. and they would still call it custom.

It’s not a “from the ground up” custom chip. Unified still requires a significant amount of chip area per die, especially if they want to have a relatively beefy GPU (somewhere below Radeon 8060S, but above Radeon 780M).

I would imagine this gives the best perf./buck from Valve’s POV, without costing an arm and a leg