Audacity is a cross-platform app that runs on (at least) Linux, macOS and windows (I suspect most other *nixes are supported to).

Also, note that most creative apps (music, video, art) do not follow the UI guidelines for any particular OS platform (partly because they tend to be cross-platform, partly because their needs are very different from office productivity and communications apps. This is true even when the apps are nominally made by Apple for Apple (see Logic Pro or FCP for examples).

Finally, Tantacrul has zero responsibility for the UI appearance of the Audacity that Wavacity is based on.

So we agree: this appication/UI has nothing to do with "Windows XP".

As a game dev I haven't seen an app (except browser and terminal) that looks like the OS native UI for quite a while.

Blender, Houdini, 3DCoat, Photoshop, Krita, FL Studio, Bitwig... all of them use their own UI widget.

I'm trying to think how you'd even make blender or bitwig look native, but then I remember the DirectShow node graph editor from the windows 98 days and I'm going to stop thinking about this again.

I strongly support using OSS every chance I have, and do what I can as a non-programmer to support OSS projects, but as someone who trained on industry standards for audio editing (since the 1990s, starting with Syntrillium’s Cool Edit Pro) I found Audacity quite difficult to use. I really don’t understand this philosophy that prefers when applications lack any cohesive visual design language. “Designed by engineers” is every bit as problematic to me as “engineering by designers”.