Being modularised allows the basic user experience to be kept very simple. It does not need to grow into something like an IDE. Hell, according to Packages  even tools like find-and-replace have been modularised so I do not think it follows that Github would carelessly decide to create a big ball of mud!
And additionally this tool has removed barriers that previously existed before.
Since it was created by Github they will be able to expose APIs to create features which are currently not possible.
Likewise the UI being implemented with WebKit means that the user interface can tightly represent what a user is used to seeing at different stages of their development process.
You might have read "The Design of Everyday Things"  before. There are certain elements which you need to control to create a good user experience: (1) discoverability, (2) feedback, (3) the conceptual model, (4) affordances, (5) signifiers, and (6) mappings. Without ease in changing the UI, and the possibility that Github will have self-interest in exposing extra APIs, it would be a lot more difficult to control for each of these.
It's just an opportunity to try new ideas. I'm not suggesting that this would be preferable to everybody.
The basic user experience does not exist; or rather it is trivially basic in any text editor.
What's important is the advanced user experience, and it simply doesn't exist yet. For instance, Python support amounts to syntax highlighting; nothing compared to python-mode and elpy in Emacs, or PyDev in Eclipse.
There's also a large gap between the promise of "Full-featured, right out of the box" and offering fundamental functionality like settings-view and command-palette as extension packages.
This project has years of hard work ahead to begin competing with Emacs and Vim in some niches; hard work that isn't going to happen because it's proprietary software, and contributors will prefer the open source text editors.
You're probably right, but even if this is not the case, they will at least hopefully have a better understanding of how to use the APIs well, given how often they work on and with their own APIs.