Nanite IS stepping around the fixed function pipeline because the triangles they draw are on the order or less of the size of a pixel. They only allow the fixed function pipeline to render the triangles when they are big (ie. almost never).
Current GPU memory models don't give you enough to guarantee progress. Most models seem to do that well enough, but there are pathological cases that aren't that uncommon.
Both of these are backed up from Nanite presentations. So, if you think those are false, know that you are disagreeing with Unreal who has put it on the record.
GPUs don't have MMUs. They have memory interfaces, but they don't have the parts that allow you to protect memory, handle system vs user protections, and manage load. This is why things like Shadertoy can bring your computer to a grinding halt or even cause you to crash.
About the only thing "false" was the fact that the graphics cards don't have a defined memory model. As was pointed out, Vulkan 1.3 does define a memory model. However, the manufacturers have yet to catch up nor are they compliant with that memory model.
Perchance you would like to revise your statement?