The use of “instant” to denote a time coordinate always rubbed me the wrong way. While “instant” arguably can mean a precise point in time (“in that very instant”), the primary dictionary definition is a (very small) time interval. This is also reflected in the meaning of “instantly”, which does not mean “simultaneously” or “at the same point in time”, but rather something like “immediately after”. Similarly for the adjective “instant”, which likewise generally implies a chronological succession, however short. I can’t quite put my finger on it, but it instinctively feels off to me to use it to denote timestamps.
In any case, it’s about how the word is normally used. In normal language you never say something like “those two instants are three hours apart”, or “three hours passed between the two instants”. In contrast, you would say something like that with “points in time”. For example. one can say “this was observed at two different points in time”. Nobody would say “this was observed at two different instants”.
