Just my opinion, but if MS wants to do an UE5 demo, it should be a completely different demo. Myriad of reasons, but just one example: as amazing as the Lumen/Nanite demo was, we've already seen it. It won't wow the same way twice just because it's running on a different platform.
Plus this way we could get two different/unique demos possibly showing off different aspects of UE5's feature set
I mean, maybe?
But this completely ignores the ML and DLSS-type optimizations in XSX's hardware to assist with those very type of things. Also I doubt 2.4 GB/s raw sequential read speed is too slow for 8K asset streaming, especially if using the advantages of Velocity Architecture and DirectStorage in tandem.
It would probably need a tad more optimization for similar results, but the results should be very comparable between the two and not to the degree suggested in your post.