I'm a little doubtful, on the basis that it would take HDD users about three minutes to load into the game (probably 4, by the time you start doing something with all those assets). I know HDDs are slow, but 4 minutes of loading is extremely excessive by any standard.

Also, on PS3 it runs in less than 256 MB of RAM... where is it loading these assets to? Most games have RAM consumption on the order of 4-8 GB in most situations, not all of that is assets, and not all of that is read sequentially.

Much less. Most gamers don't have 20GB VRAM GPUs. The game doesn't use 20GB RAM either, I think around 6-8 GB. So there's just nowhere to keep 20GB of loaded assets.

But not necessarily as a sequential read.