Alpha and Beta are stages of development
Demos are playable uhh, objects, for lack of a better word.
You can have a demo that was created during any stage of development, as long as you can make it playable.
Beta is almost always playable, so they probably just sealed off any glitchy areas.
It’s like a steak maybe (or maybe I’m just hungry, tbh.) You can have rare steak which you shouldn’t eat maybe (alpha) but anything after that is theoretically some level of doneness that can be served. As long as you’ve likely killed all the bacterial by bringing it up to a certain degree (that would be the 'closing off unplayable or glitchy areas bit) then it’s fine to serve.