Explained multiple times already even in that thread. SoD run in backward compatibility mode which cannot use velocity architecture. Exactly same thing will happen on PS5 if you even will be able to run your game.
It depends on Sony's API. Most of Sony's "Velocity Architecture" is baked into the hardware and can't really be turned on or off. It's just how it works. From reading the Sony Storage Architecture patent filed in 2017; if current PS4 SDK API calls for file loading just ask for a filename and where you want it, it would be trivial to wrap calls to that into PS5's File Archive API that would leverage the entire PS5 IO stack.
For File Archive API you still just ask for what you want by a path and filename. That then gets handed off to a co-processor which hashes it and performs a look up in a table in system RAM to find the logical block address (LBA) and the rest is pure silicon transistors flapping around, with the CPU completely hands off until it gets notified the data is now there and ready.
There's really nothing else you need to do from what I can see.
The architecture is also highly targeted towards random read performance, even to the point of storing its LBA to (Physical Address) PA look up tables in SRAM in the SSD controller, as they deemed even DRAM to have too much latency according to the premise in the patent.
If PS5 handles PS4 file IO calls through File Archive API instead of a traditional virtual file system, then load times on PS4 titles should be massively quicker than before, and be under a second to fill how much RAM a PS4 title reserves.
Anything that gets written to the SSD gets compressed in Kraken format, regardless of what compression was used on the source disk. Again it's hands off and part of the pipeline.
The entire point of using an API for something like file access is that what happens underneath is abstracted and its implementation changing doesn't matter so long as the interface (the parameters you pass in, and what comes back) remains the same.
This is precisely what Cerny meant when he said the best part is as a developer you don't need to know how any of this works. You just request your data and a few milliseconds later, it's there.
It will all hinge on whether PS4 backwards compatibility games will route file IO API calls through FIle Archive API or whether they're maintaining some legacy system. It might even depend on a game by game basis, as with Boost mode. It would be trivial to do, though. Maybe it could break some loading screens or "cut scene" load screens and it's on a game by game basis. Nothing would need to be rebuilt or recompiled though, just where the API calls are being redirected.