Yeah, a 2 month development cycle is insane. Then again, it's basically a mod on top of a game with 6 years of development behind it, so I guess that probably helped matters quite a lot. Epic also has tons of development experience under their belts, know how to write solid net code, and know the engine like the back of their hand (at least I hope they do, lol).
Save the World is supposed to be free-to-play when (if?) it launches. One of my friends plays it regularly and he really likes it, so...I dunno. The main reason I'm hesitant to buy into it is because I fear that on top of the initial asking price I'm going to also have to deal with microtransactions when the proper launch happens. I generally try to avoid buy-in betas for future F2P games for that reason. There's no telling how the company is going to reward your loyalty, if they even bother doing so.
Meanwhile, Fortnite: BR is free. I dunno, maybe I'll give it a go one of these days. It wouldn't cost anything aside from a bit of time, after all.