I'm not sure I understand OP's point about travel time. Today, DL offers 22:10 as the total travel time on JFK-NRT-SIN with a transit of 1:25 at NRT.

Now, obviously as winter headwinds pick up, I would expect overall westbound travel time to increase. Indeed, come October, the total travel time JFK-NRT-SIN increases to 22:40 with the same 1:25 transit at NRT.

By the end of October, though, that increases to a scheduled 23:55 total travel time JFK-NRT-SIN with a transit of 2:05 at NRT.

Starting in December, DL actually has the NRT-SIN portion of DL 621 scheduled on the 332 some days and has some inconsistent arrival/departure times, but the same general schedule of right around 24 hours with right around 2 hours at NRT remains.

So, all that said, Delta hasn't done anything at all to make JFK-NRT-SIN a more efficient connection from a time standpoint. Those changes all fall within the usual adjustments that go along with tweaks in the schedule at NRT. Most likely, they also haven't made a decision to become the price leader in the market, but OP is just looking at a time where they happen to be the cheapest. Look again in a week (or an hour, for that matter) and the fares could be completely different.