With decompression enabled, they're claiming to be able to get 9 GB/sec from the SSD.
They get 448 GB/sec from RAM.
Streaming in assets, yes, will be a little faster, but look at the example - streaming in a whole neighborhood in the blink of an eye is an absurd level of overkill because you can't even remotely display all of that at once. They'll probably still require prediction to figure out what to load in, and be able to load more important stuff into RAM rather than having to load "expensive" assets in case they're needed, but it still has to get into RAM at the end of the day.