I've seen several applications made with Godot, and I've even tried to make one myself. It's possible, but in my opinion the GUI workflow isn't rich enough yet. The article mentions the positives (and there are plenty) but things that should be simple like adding fonts or styles are still awkward. But it can be done - as mentioned Godot's own application was written in the engine itself.
I also don't know about GDScript as a language to do anything serious in... maybe focus on C#?
GDScript is essentially a slimmed-down Python with all the bad dev. experience stuff removed (version- & dependency management that is).
Godot has C# support since 3.0[1]. Though to me C# seems as arbitrary as any language – Godot doesn't strive to compete with Unity.
Why not use Scala or Lua?
While I'm not a fan of Python I think its a reasonable choice given the target audience of devs using a game creation IDE/editor.
Experienced devs who want to use the Godot ecosystem/frontend/engine can always use whatever to write their game in by adding the resp. bindings; e.g. Rust[2].
GDScript uses significant whitespace but I'd hesitate to use it and Python in the same sentence. The similarities are only superficial.
And the problem with using any language not directly supported by Godot (anything but C# and GDScript) is depending on a third party to provide language support, and keep it updated with the engine. Rust bindings look like they have effort put behind them, but who knows? The only thing you can depend on is GDScript - even C# support seems like an afterthought (although I think access to Nuget and libraries make it a better choice for application development.)
I guess that's an effect of Electron & co.
The 3D apps we used in the 90s had GUIs of similar complexity, all drawn in app (not using OS widgets).
They were considered 'large' when they approached 5MB.