A rectangular form factor for display has been the standard for decades. All applications, save for some desktop widgets, are shaped accordingly.
Putting rectangular designs on this would waste lots of space at the corners, and I don't see a ton of developers jumping to go support a triangular display for their UIs.
They parent replies that nobody would want triangular tablets because of the waste at the corners when drawing square shapes, and the well-known rectangular nature of existing displays means nobody would jump at developing triangular UIs. They completely missed that the point of the triangular notepad is that it unfolds. That implies they didn't look at the article, because it has a picture of the triangular notepad doing that.
When I thought of a computer notebook doing this, I was envisioning a keyboard on the bottom like any other netbook.
I suppose in the tablet sense it might be possible.