While Windows UI changes a lot, all old apps work fine in Win11. I guess the author was referring to Wayland bull…t

“Have to update” is the key here. You can still run apps from windows 95 if you want to.

I can run X applications from the 80s on Wayland

meanwhile Windows 11 has 4 or 5 different UI styles in control panel

I don't run Wayland, but a few times per year I find myself picking up some unmaintained X thing and running it with a recent OS. I'd say on average compatibility is much better on Windows.

Binary compatibility on Linux is often out of the question. Frequently this means picking up some old libs. libc5, old stdc++... For really old stuff with fewer dependencies that may not be a problem. As you get into a more modern era where software started to pile on large heaps of dependencies it becomes more challenging.

Source compatibility typically means porting, sometimes nontrivial. Likely something is written in a time capsule of that era's poor C and C++ standards compliance. (i.e. C89 or C++98 existed, but compilers of the day accepted lots of nonsense, so software of that era doesn't even conform to those.)

In contrast the Win32 API or COM is designed around binary compatibility. Maybe early 2000s dependencies (when MS started getting worse at this) are a problem. I think Win16 on modern amd64 is also a problem. But on average, compatibility is higher.