Your observations are correct. But as Jenny said, the Gaze SDK time sync functionality and the EyeX Controller firmware is experimental and not adapted for the kinds of applications that require careful time sync.
If you want to have full control of the clock synchronization, you should take a look at Tobii’s product lines for research and analysis. You already mentioned that you have access to an X120. If you use that together with the Tobii Analytics SDK 3.0, you should have the control you need.
The Tobii EyeX product lines are primarily meant for games and consumer applications. We have not found any use case yet where this kind of careful time sync is required. But if we find that many of these applications require time sync, then we’ll have to add it.