You're right, XP SP2 does support SATA drives. So does XP, XP SP1, and Win2k.
It's still the ATA interface at the basic level. The changes are mostly physical, and I believe there's some signaling changes to go with theem, but SATA is still ATA, so we're not talking a brand new storage interface from the ground up. Therefore, SATA does not require XP SP2. However, with the 48-bit LBA barrier, the hardware, which has been addressed for a few years, requires a software compliment. That's why XP out of the box requires SP1 or later to properly recognize the full size of any hard drive.
If your thinking was correct, I couldn't have installed Win2k on my 80GB SATA drive...at least without drivers, which I didn't need. Had I had a RAID array set up, I would have, but that's very common.