Intel mandates the use of faster LPDDR5X. You can have DIMMs, but only with lower-performing GPUs that would probably have been slower than the Intel and AMD hardware Framework was already shipping. But Framework also clearly didn’t want to give up modular RAM, something it did to make the Framework Desktop work (AMD similarly requires LPDDR5X for that desktop’s Radeon RX 8050S and 8060S).
