So yes the Samsung 980 Pro is faster than the Playstation 5 SSD, the PC has compression technology as well, it's just not baked into hardware.
I'm still curious to know what would happen if I inserted a "cheap" 3-4GB/s ssd into the PS5. Would it be refused or would I simply wait 2.56s instead of 1.45s loading a game?
This has already been discussed here. If you have lastgen PS4 games, those will be playable directly from any attached drive, since PS4 games don't expect to have high speed SSD and can run just fine off of slow disk drive. You can also store pictures or videos on the slow drive, but PS5 games require being installed on the high speed SSD to play, because that is the baseline expectation for nextgen. Some SSD cards do meet the speed requirement to play games directly off of them, and if you install one of those SSD it will work just like the built in SSD (except giving you more room).
If an expansion drive (SSD or hard drive) doesn't meet Sony's spec (which is actually faster than Sony's built in SSD because reasons) then you can still STORE PS5 games on it, but you can't play the game from that location. You will need to free up space on the built in SSD (or other qualifying SSD), possibly first moving another game FROM the built in SSD to the slower drive with plenty of empty room, and then move the game you want to play from the slower drive to the fast drive so you can play it. Any SSD even if not up to Sony's spec should make the copy process alot faster than a slow hard drive, but it will take a certain amount of time (~10 minutes? this depends on game size and speed of expansion drive) before you can play the game, more so if you want to first move game FROM the fast SSD to the slow storage.
Think of it like you have a stove and you can instantly cook anything on it's 4 burners. You actually have more food than fits on 4 burners and you keep that in your refrigerator. But you can't immediately cook it in the refrigerator, you need to move it to the stove to cook it, and if the stove is already full you need to first move one item into the refridgerator.