PS3 on PC emulation is with no documentation, however, and it's very impressive.
Meanwhile, Sony has all the information they need regarding how the PS3 ticks, so emulating it is more plausible.
If they're gonna migrate their PS3/2/1 library to the new Azure PS Now service, whenever it comes, then they will need working emulators for all those systems. If that's the case then porting it to PS5 is a possibility.
I think the reason they're only talking about PS4 backwards compatibility is because it's a dead cert and it's close enough to 100% that they can say with confidence that games will move forward. With pre-PS4 hardware it's probably done on a game-by-game basis that's reliant on it working both technically and in a business/licensing sense, so they don't want to announce it only to have players disappointed when certain games they bought just for this don't actually work.
The way I see it is when PS5 is closer to release they'll tell all, including what games will or won't work.