Whole I agree that this is UX design, but I suspect that UX can be (and often is) a product of the type of organisation producing it moreso than the "UX designer hired to do UX." UX for something like Gimp is very hard. The metaphors have to encompass a ton of stuff and the UX needs
A web app created by a bank, electric company or somesuch will tend to be different from an app created by an SV-ish company in a pretty predictable way. Similarly, Gimp seems like a relative of other GNU/OSS software... Calibre comes to mind.
Different types of organisations or ways of doing things have different strengths and weaknesses. Th