the XONE is not hUMA, because is UMA, and being UMA, is not necesary a 'downgrade' to be hUMA.
hUMA is a patch for UMA, that allows two or more memory busses to share a single memory pool and allow cache-coherency between the Proccesing units attached to that busses.
In this case, que GPU and the CPU of PS4 uses the Onion & Garlic busses. Onion for CPU, and Garlic for GPU. Onion has a MMU but Garlic lacks one. Onion/CPU can implement VIRTUAL MEMORY POOL, but Garlic/GPU only sees PHYSICAL ADDRESS/MEMORY POOL. this way, PS4 can use PRT, but only with PHYSICAL LOADED TEXTURES IN RAM. if the PRT engine need to 'swap' or to load new textures, needs to do by SOFTWARE.
the XONE is UMA, and access to DDR3 pool is controled by the CPU MMU. the MMU uses the ESRAM as a L3 cache for GPU memory access. this way, the GPU can view the VIRTUAL SPACE ADDRESSES offered by the MMU, and can issue directly 'page faults' from the PRT engines, allowing to create and texture worlds 'bigger than RAM'. the page fault from the GPU access is caught in the MMU, dispatched by the CPU, and requested via DMA or DME to the disk (virtual memory subsystem).