Transfer the whole game from HDD to the SSD before you can play it?
Hahahaha.
Be serious. That would be hyper unintuitive to the whole fast booting games.
The reserved space on SSD makes alot more sense than having to transfer a ton of data to the SSD before you can actually play the game.
As has been stated game data is more modular on PS5 so when you boot a game even off an external it can transfer whats need for boot to the SSD reserved space, then slowly(as quickly as the HDD can go) fill it up with whats needed for the game. And from there the game can leverage the SSD quick-switching to/from main memory. So for fast travel maybe all the fast travel points are on the SSD when you fast travel thats quick switched into memory and the rest of the area is then loaded into the SSD reserved space.