Word of mouth must have exploded because of PS+. I assume they would have simply had a slower build up if not for PS+, but I dont imagine them having the fast success they have now. It feels like PS+ helped them capitalize on the momentum at great speed.
You have a friend who downloads it and loves it. He pesters his friend to try out this new indie game that he would otherwise not even look at. He gets hooked too. It snowballs.
PS+ removes the barrier of making a purchase and since online gamers need it, it becomes that much easier to download.
And that's all they need. The chance to get it into people's hands. Way too much skepticism by people who see Rocket League as another indie downloadable. And for good reason. There are a lot of indie games that are fun for like 5 minutes. No way would people put down $15 like that in my opinion.
I also could have sworn that Sony had to pay a little more for Rocket League, but that could very well be me being mistaken. I dont think it was originally supposed to be a PS+ game, was it?
In any case, I hope Psyonix is happy with the end result and that they can continue the momentum. Retail is perfect especially for certain regions and countries.
I am hoping one day we see awesome licensing for the Simpsons, Futurama, Star Wars, whatever.