There are a lot of low-level intrinsics that need to be called behind the scenes to do the recording, and I'm not surprised that they did their first experiments using the 32-bit versions, since they might be better documented, or there might be more expertise on the team if they've been working on 32-bit low-level stuff for decades. All those intrinsics should have 64-bit versions, though, so it's just a matter of time before 64-bit support lands. I'm sure they'd welcome pull requests!