That's fair, and at best bonus of the patching process. Cerny did specifically call out that the PS4 process recreates files in their entirety to prevent additional seek times, so it can't just be patch installation insurance. Unless... you're hinting at something called out in the SDK docs that contradicts what he said in the road to PS5 video and I'm too dumb to get it? :D
Nah, the SDK doesn't talk about it (as far as I'm aware, plus I'd never publicly talk about stuff explained in the SDKs). This is just through educated guesses. My reasoning is the PS4 and 5 share file system behaviors with older FAT systems, you cut power to the console and it needs to validate the drive. Thus, I know it has no journaling, and we know the update procedure works similar to how Steam used to work back in the day, where it needed a copy of the game to patch it so it could never get tripped up by the live game or sudden power loss.
Defragmentation can't really come into it, because I also know you need the largest object in free space to be able to move anything around, not just the thing that's fragmented. Stands to reason then that it's not related. The thing Cerny points out doesn't really make much sense in that regard, unless they just mean individual files, of which that would certainly mostly work as consequence, though it wouldn't be perfect. However the individual game files being unfragmented isn't really as helpful, you'd want it to apply to the entire contents.
On PS4 it doesn't require the entire size of the game that it needs to update as free space. It only needs the free space of the segment/s it needs to update. It can be the whole game or it can be just a small segment.
There seems to be conflicting reports then. People definitely report you need the whole game in free space, is that not the case, do we know for certain? But again that wouldn't really defragment the game as a whole, in fact it wouldn't do that at all in that case, the files would slowly split apart all over the drive.
