If I were one of the internal tools programmers, I wouldn't be "extremely disappointed" by this leak...
I'm seriously impressed by the quality of the internal tooling. All that effort only to be seen and used internally. And yet they still polish the UI!
Most of the internal tools where I work look terrible, unless they're web based where some framework can do the hand holding.
They are using imgui. It would be hard to find a game studio that doesn't.