I would say it is the controller. Because even old PCs with slow HDD used that for "cache" as some virtual RAM.
So when SSD is very fast on PS5 it can help the RAM without being an issue for the performance I guess. There is also the coherence unit that will support the fill and removal of data from the RAM and that is on the SSD solution.
Yeah, it feels like the speed they went with is what they found to be necessary for these functions. I believe he said something like that too.
To eliminate loading screens, the new decompression technology to save dev time, not having to repeat assets clogging up space on the Blu-Ray, and to function as RAM for better games.