Sort of, obviously quite a few games are optimized for the PS5 and Xbox series X.
GPU cores are generally identical between the iGPUs and the discrete GPUs. Adding a PCIe bus (high latency and low bandwidth) and having a separate memory pool doesn't create new opportunities for optimization.
On the other hand having unified memory creates optimization opportunities, but even just making memcpy a noop can be useful as well.
GPUs are all about (compute & memory) bandwidth. Using the same building blocks of compute units doesn't yet it go fast. You need a lot of compute units and a lot of bandwidth to feed them.
The performance dependency on DGPUs doesn't come from the existance of a PCIe bus and partitioned memory, but from the fact that the software running on the DGPU is written for a system with high bandwidth memory like GDDR6X or HBM. It creates opportunities for optimization the same way as hardware properties and machine balances tend to, the software gets written, benchmarked and optimized against hardware with certain kinds of performance properties and constraints (like here compute/bandwidth balance and memory capacity, and whether CPU & GPU have shared memory).
GPU cores are generally identical between the iGPUs and the discrete GPUs. Adding a PCIe bus (high latency and low bandwidth) and having a separate memory pool doesn't create new opportunities for optimization.
On the other hand having unified memory creates optimization opportunities, but even just making memcpy a noop can be useful as well.