Interesting... That tweet is the first I ever heard of UWP apps being distributable through 3rd party stores, and I don't see anything in the link that he provided that supports his claim - its all in reference to the Windows Store.
It is interesting if true, but the devil is in the details, and Sweeney points out the similarities to Android, where you can technically side-load applications, but it is a pain in the ass, and I'm sure 99% of users couldn't be bothered to do so. Look at how much trouble Amazon has had getting people to use their app store on anything but 'Fire' devices.
That's not what that says. It says you can use more ways to put things in "Windows Store", not use UWP APIs from programs not sold through "Windows Store".
UWP is a fully open ecosystem, available to every developer, and can be supported by any store. Broad range of tools http://bit.ly/1QIHTf0