Nonsense.

Make a thought experiment and imagine that each new major GTK (or Qt) version had a different name. You'd still have old GTK, which wouldn't be maintained anymore as the devs moved on, and new NTK or whatever, which would be what was still developed and used these days. Somewhat similar to old GTK since that was its roots, yet distinct.

And that's exactly what happens, they just reuse the name because well, why wouldn't they. You can still use GTK2 just like you can use Win32 API, but nobody does modern apps in either GTK2 or Win32 API or expects them to smoothly blend into modern desktop these days.

Not even speaking about the fact that The GIMP is a very special case that makes it probably the hardest GTK app to port to newer versions in existence.

Even if GTK2 were to get hidpi support, it would still require making the same types of changes in the applications to support them that would be required to port it to GTK3 and get hidpi there anyway. Only the most trivial applications without any custom drawing/widgets would have hidpi working without any changes. Rule of thumb, if the app's code uses raw pixel units anywhere, that would need to be modified to account for scale. You would probably need to modify all the GTK2 theme engines too.