Yes, they were not yet tired enough of sitting, at least not enough to stand up for some small amount of time, only to have to sit back down again. This is true

Yes, they were not yet tired enough of sitting, at least not enough to stand up for some small amount of time, only to have to sit back down again. This is true for any part of the flight, not not just before the start of descent. There are also any number of other reasons those individuals may not have been motivated enough to stand during the flight itself, as a sibling comment pointed out. My point was that the original observation was not actually of anything odd but, instead, of something consistent with this particular motivation.