You didn’t read the webpage it sent you to, did you?
Basically, it seems that a little while back, the Devs made the kit available through the Epic Games/Unreal Launcher, and it’s the easiest way to get it going now, after making a (free) Epic Account. That being said, you have to redownload it all if you go that route, which if you have a slow or capped internet, I can understand you doing. If you look around here more there’s instructions on getting the Steam version going, though it’s a little more confusing and annoying.
I did the same as you tbh. It’s pretty annoying, but that’s how it is right now.