It may have been done with Wayland in mind, where there is no client-side way of setting the window position, or even querying it, since wayland does not let the client know. The position of windows is managed entirely by the wayland server.
While I do have Wayland in mind, that's specifically for what I can do with the GTK+ backend when running against GTK+'s limits. For this particular feature, there's a different rationale, but I can't remember what it was right now. That being said, I may add such a thing in the future (there's something in futureplans.md about centering the window right now, so...).
And yes, this is still rather incomplete (I'm surprised it's getting so much attention this early!); please feel free to make suggestions!
