Don't confuse the I/O unit which is inside the SOC with the controller which is outside the SOC.
The I/O unit, which resides inside the SOC, has all the cool stuff like the decompression block, SRAM, coherency engine, etc. while the controller is outside the SOC:
If you look at that image, the green line from the flash controller to the SOC is x4 PCIe 4.0 lanes. There are 4 lanes that go to the expansion bay too and they will route the data from that external SSD to the SOC the exact same way and it will enjoy the same I/O unit that's inside the SOC. What that external SSD won't have is the controller, it will have its' own. That's why the external SSD will have to have ~7GB/s raw bandwidth or higher, in order to compensate on thing the PS5 controller has like 6 level of priority.