Speculation (like all of this), but I don't think this is necessarily accurate. There have been plenty of times where I've started an oGCD action, but then it was interrupted; in other words, the client starts playing it out, but then the server says "nope." This seems to indicate the client is trusted, until the server confirms (or in the case of my example, rejects) the action.

All that said, I'm all for improving latency hiding techniques.

Also worth noting for discussion... FFXIV's tickrate is really, really slow. It's nothing compared to what you get playing an FPS or MOBA or similar.