The very fact that Sony XDev is asking on Twitter 'what PS2 games would you like to see on PS4?' heavily implies this will be digital releases of PS2 games rather than backwards compatibility for discs.
I'm guessing the emulation isn't robust enough in the general case for them to simply release it with disc support and claim (close to) full backwards compatibility. So they'll cherry pick some titles, make sure they run OK on the emulator, and release them digitally.
Or their solution may be similar to Microsoft's - I. E. Although you're playing the PS2/360 game as-is, Sony needs permission from the publisher before they add the game to a BC list.