I guess they are using very specific machine instructions to achieve their ends.
On Linux, the library uses the GCC atomic intrinsics. As such, the library funamentally supports any platform GCC supports with its atomic intrinsics. So the instructions are indeed machine specific, but they're provided by the compiler through a standard API.
Of course, actually getting something to work means actually compiling in on a given platform and seeing the test programme pass.
("Do you REALLY want to use that API? I think you should space your comments out a bit more. Hey, your battery is getting low! BTW, I've just emailed your entire source base to Google for assimilation.")