While this is true that on Windows 64 the sizeof(long) is 4 while on 64-bit Linux it is 8, and while it may have been a problem for coLinux (I don't know how it was built, however I doubt it was with much MSVC anyway because the Linux codebase uses tons of GNUisms), and don't think it was very much relevant for WSL1 (they only had to implement the syscall APIs correctly), and probably even less so for the core of WSL2. WSL2 is basically a VM plus some integration bits (think kind of like your VMWare / Virtual Box integration tools) -- this may be tricky for the DirectX forwarding, but I'm not really following the recent devs and MS made it work already anyway (not upstream, but they have the code and they posted it on the LKML, so you can take a look at it) pretty much with a gigantic copy past of tons of data structures, so they somehow managed to handle that.

And I suspect it was not even too hard anyway: identify from which universe comes a type, then identify where you need compat, and use an explicit size to setup the typedef, and it's ok. Windows typically uses uppercase defined types like LONG, and Linux does not do that, so it's hard to mix up unintentionally.