Ah, it looks like what they are fixing is the "rolling sunset/sunrise" effect. Ever notice how if you aren't looking at distant chunks when the sun sets, as soon as you do
look at them you see them lit up as if it's daytime, and then a moment later they go dark? And notice that they do this chunk-by-chunk, so it's quite noticeable and not at all seamless.
It sounds like they have found a way to deal with this so that the day/night lighting changes are no longer chunk updates. This would make them instantaneous, if I am understanding correctly. Cool!