Yeah, now that quelsolaar clarified, I am fairly certain that claim is not true, for exactly the reasons you describe. Of course, there is also the practical example of the mingw compiler, which does implement C11 for Windows, as a counter-example to their claim that it cannot be done.

So, there is probably a kernel of truth, as far as I know the DLL model in Windows is equivalent of RTLD_LOCAL, so global variables are actually instantiated per DLL (but of course not shared cross process), which for example makes allocation behave differently. So a spinlock pool between the main program and a DLL wouldn't be shared, making cross-DLL emulated atomics problematic. But I guess there are ways around that or simply the expectation is not to share non-address-free atomics across DLL boundaries.

In my experience, being similar to RTLD_LOCAL avoids a whole slew of sharing/unique accidents compared to the pile of hacks that is ELF. It is sometimes both the hardest and easiest platform to work with since it is the conceptually most consistent but also therefore the most primitive linker. But that is just not an issue, as the compiler must work anyways to ensure atomics work correctly per the platform ABI.

Indeed the problem is otherwise not restricted just to memory sharing: even the particular CPU instructions chosen can mean one compiler is incompatible with the output of a different compiler when it comes to atomics even when locks are not involved (the specifics of which barriers are used and where often mean there are multiple valid, but mutually incompatible ways, to emit atomic instructions)