I think the key was that the gleaning you mention was collection of unharvested _leftovers_, whereas the article talks about waltzing into someone else's yard and taking things off of their trees. I didn't notice the author implying that it was limited only to trees/plants that they knew were in a post-harvest state - it seemed rather the opposite.
Might depend on your area of the world but around here you've only got a few days between the fruit ripening and the birds eating away at it, so ripe fruit on a tree is fair game.
