Startups can merely fully dedicate themselves to exploring new spaces that open up because they're not burdened by things such as being already occupied with doing something, especially something that's profitable.
So when something new appears like VR/AR a startup can jump right in at first opportunity without having to think too much about it. This has the potential to be an advantage and means that startups are "faster" (really just earlier) to certain things.
Github is a profitable business that has nothing to do with editors, so I'd argue that it's not a startup anyway, certainly not in any far that gives them advantage in developing editors or IDEs.
Quite frankly I really don't get why they even bother. It doesn't differentiate their actual business in any useful way and it doesn't make money. In the meantime they're actually facing competition from Gitlab, a company that's adding features so powerful you can and other people have build entire business around them.