OED late sense 3 - Of the time of day: at or till a late hour (first citation 1400).
So there's no reason to suppose "We talked until late" (or, for example, "Go to bed! It's late!") are in any way "shortened" or "informal" versions of something longer and more "grammatical".
I would just say that "We talked late" (without until) sounds slightly "odd" to me, but that's probably because I assume the talking started earlier, and continued until late. I see nothing unusual about "We ate late", because I assume it means we didn't start eating until late.