The first difficulty is that, according to this view, a human being (or any ultimate subject of existence) is the sum of time-slices suitably connected (say, by biological or psychological continuity). But what is a time-slice, and, how could time-slices give rise to a human organism's (or any organism's) extension through time? If the time-slice itself does not have temporal extent, then the addition of any number of time-slices to each other will not give rise to a temporally extended series -- just as the addition of any number of unextended points will not produce an extended line. On the other hand, if the time-slice of a human being does have temporal extent, then no explanatory gain has been achieved by denying a persisting human being, since one will then (by necessity) have admitted that an individual as a whole can persist through at least some extent of time. But if one must admit persistence through time at one level, why not admit it at the level that common sense and explanatory practice seem to demand -- that is, the lifetime of a human individual who persists through time?
I am under the impression that David Hume, with his theory of sense impressions, inspired the view they criticize here. If any reader can confirm or deny my impression, please do so.
George and Lee's essay does not cover the phenomenon of twinning, which seems to be what their target has in mind at points. Their literal use of the word "individual" would of course run into a bit of trouble when applied to twinning, for at that point there really are at least two possible individuals within the whole organism. I am also curious how they would interpret the concept "human organism," and relate it to "human being."