Earlier quoted context omitted.
True, but most GUIs are built with the mouse in mind, and at best slap some ctrl keybinds on as an afterthought. Hamburger menus and flat whitespace are cumbersome.
Is it better to have nothing instead of a hamburger menu like this app does (according to its screenshot)? Also, what isn't flat in this app?
Flat... Whitespace. In modern graphical applications, nothing is broken up in your field of view, everything is an object on a slate. There's no organization, no context grouping objects together that help you understand how to interact with them, they're not ergonomic. Contrast that with this, while it's not a perfect example, there are clear borders, clear sets of interactions, the mind can formulate a procedure by looking at the organization of everything in front of them. It's almost like fonts, some fonts are better for readability, others are just ideas people had that might help that don't work out, and others are just there to look fashionable. UI maps and interactive flows can be easy to understand at a glance or look pretty but not give the user much of a clue what to do with what they're looking at.