Well, if you're using Linux, you can look at the sysstat code, which probably does some kind of ioctl on the raw device, and so you could look at the Linux kernel, which might end up telling you that it counts the number of blocks sent to the IO controller for writing; whether or not that means the device actually wrote those blocks, I don't think one can say.

Now it could be that the disk and its controller keep track of a statistic and it's possible to get that value, but I doubt this is what sa/sysstat relies on. Further, I've written SCSI drivers before, and if this feature exists nowadays, it's not in the standard, meaning it's on a device-by-device basis.