It's unlikely to be literally connected over PCIe when it's on the same chip. It just looks like it's connected over PCIe because that's how you make peripheral

It's unlikely to be literally connected over PCIe when it's on the same chip. It just looks like it's connected over PCIe because that's how you make peripherals discoverable to the OS. The integrated GPU also appears to be connected over PCIe, but obviously has access to far more memory bandwidth.