It's painting stuff in a canvas and not using the standard web controls.
It's the web in the sense that almost every UI toolkit also offers a canvas (for example Tk does this) and you can reimplement every control using it. That doesn't make your new toolkit in a toolkit Tk.
But why would an app that writes to a WebGL canvas be any harder to maintain than an app that uses HTML controls? If anything, it'd be likely to be easier to maintain -- we (the web standards community) are much more likely to change how a particular element is supposed to work than to change WebGL in an incompatible way.
But the framework? That will have to be updated when each OS the framework runs on changes.
The HTML controls are used by a ton of stuff, so they're quite likely to be updated. Your framework? I have to really, really trust you that you'll update them.
Look at how Motif or GTK or AWT or Swing or Tk widgets look on modern OSes. There's an entire graveyard of cross platform UI libraries out there.
