I expect it won't include the Scrub Abe skin or Alf's Escape either. Considering Oddworld is starting to gain more exposure again, the free PS+ version could certainly help introduce gamers who wouldn't otherwise have bought the game- and those perhaps on the fence about giving it a go. If and when an Exoddus remake comes along, there would be a lot more gamers who played New n' Tasty and loved it, who would buy the sequel day one.
I have friends who like Oddworld, have played the original games when they were kids and enjoyed them, but would never be interested enough to bother to buy the new ones. But they'd definitely download them for free! If they get hooked, I'm sure they'd buy the DLC and Exoddus eventually.
As Shade667 said, Sony offer devs something in return for the promotion, which JAW/OWI will have agreed to. Not to mention it bumps Oddworld back into the spotlight again. I'm sure the promotion will be worth it