Can your GUI system build a Digital Audio Workstation? It doesn't matter how good your text handling is if everything is too unresponsive.
Can your GUI system scale arbitrarily when I put it on a humungous monitor? It doesn't matter how good your text rendering is if I can't read it.
Every single GUI system currently fails both of those.
GUIs are currently stuck in a path-dependent local minimum that needs to be broken out of. It's not clear what the path forward is, but this article simply reinforces the broken status quo.
No, you don't have to render HTML. No, you don't have to support application tabs. No, you don't have to support modal dialog boxes. And, no, you don't have to behave like native. You don't even have to support an "editor".
About the only part I agree with is "If you support interactive text, it's a nightmare." For most people, this needs to be outsourced to a library because you will never get shaping, rendering, and input methods even close to correct.
In fact, I might even go so far as to say that "99% of GUI misery is supporting interactive text--don't do that." These are the "embedded" GUI systems that the author so blithely dismisses in the first couple of sentences.