: And no one at 343i has mentioned going gold. :P

This is the key thing. Reading it in that light (as I read it the first time having already seen the 'ad'), it feel very end-like; someone saying it as a final thanks and farewell really could use those very same words.

Without the ad, it may have felt different right off. But man, I hope it's not that; unless they've already finished recording his vocals for Halo 6 as well :P Still...