The Win32 ABI is also just a wrapper on the native API, which is only stable in practice, but not officially according to any Microsoft documentation.

Glibc is userspace seen from the perspective of the Linux kernel.

It doesn't really matter if it's 'just a wrapper', because said wrapper provides an ABI. Even if the underlying Native API changes, the interface the wrapper presents to other compiled binaries won't. The latter will contain caller/callee register setup, type layouts, function arguments and more for that wrapper.

Cygwin is also 'just a wrapper' for the Native API and Win32, and look how drastically it changes the ABI of applications.