Memory on an expansion card isn't gated on PCIe 5, it's gated on CXL support. CXL and PCIe use the same electrical/physical layer but the protocol is very different.

The problem with putting (system) RAM on a PCIe card is that PCIe is not a cache-coherent interconnect. If you have a cache line that resides on your GPU sitting inside your processor's cache a remote modification to that memory by either the GPU, another CPU core or some other PCIe device with NOT invalidate the CPU cache line. You also have the fun situation that if it's modified on both ends simultaneously the resulting state will be non-deterministic.

Device drivers have to be very careful about synchronization when accessing memory-like areas on PCIe. CXL adds a cache coherency protocol among other things, so that invalidations and snoops can be exchanged over the interconnect.