@ FadingTruth Thank you for clarifying. Remember, I am a graduate of "Watchtarded University".
By "entire dig site", I mean Kathleen Kenyon's entire dig site.
It looks as though I have confused the two relevant destructions of Jericho. The massive walls fell a thousand years before Joshua, and a bronze-age "backwater" fell 150 years before Joshua. The grain dates from the later destruction.
The double city wall Garstang associated with the Israelite invasion in about 1400 B.C.E. in fact dated to the Early Bronze Age some 1,000 years earlier. The destruction of Garstang’s City IV, which he had dated to about 1400 B.C.E., occurred, according to Kenyon, at the end of the Middle Bronze Age, about 1550 B.C.E.14
I didn't see Wood's additional note about carbon dates, because it's not in the actual article. Nevertheless, from what I have read in other sources these earlier dates are stronger than ever, based on the latest calibration curves.
I still think it's a stretch to say that the grain could only point to a short siege and a non-plundering conquerer. The grain discovered and tested was "charred" - surely, even a plundering army wouldn't want it!
So... what I should have said is: Based on some grain, dating from 150 years too early, the Watchtower concludes that the bible story is true, even though the "walls of Jericho" fell a thousand years before.