10 years after this, its still about the same... which is probably why an article like this has any controversy about it instead of being run-of-the-mill.
Retained mode is probably more common for user-facing GUIs, though.
I've been working in (mostly AAA) game engines and tools since the mid-2000s, largely in custom engines and i never encountered Dear Imgui, so i disagree with the "absolutely everyone". Pretty much every engine i've worked with (and didn't make myself) uses something like wxWidgets, Qt, MFC or some custom toolkit for the tools and custom stuff for in-game debugging (usually a console for keyboard use and some "page/screen" based reporting/lists/menus that are easy to navigate with a gamepad).
I do know that some game engines use it, but it isn't as universal as you think.
Qt, WPF, wxWidgets etc... was a good option up until 2015 or so, but since then the least painful way for writing integrated debugging UIs and standalone inhouse UI tools is Dear ImGui.
I'm showing my age here, but when game companies used to hire people to do tools, or leverage internal knowledge, it was often things like MFC in Windows... or the ones you mention. They would often run outside the game, often displaying lo-fi graphics.
The only "integrated" tool back in the day was a Quake-like console.
But eventually people started demanding tools that were more integrated, and immediate GUI was in the right place at the right time.