There is some level of toxic that doesn't kill, it harms but theplant (or animal or fungus or whatever) can deal with the poison and live through it all.
And so as you say, he is discussing the level that harms, but doesn't kill a plant.
Roundup is really good at just a little really working over a plant; but it also is only effecting a very short time after application. If it is terrible hot, terrible dry, so the plant is not actively growing;
And if the plant is hardened up by being old, setting seed, waxy, or otherwise so the Roundup doesn't penetrate in to the plant itself;
And if the water used is very hard water, or dirty with clay particles in it, or otherwise poor water;
And of no AMS (only need 3# per acre) and/or surficant (to eat through the wax) is added to some of the cheaper formulations;
Then the plant can deal with the very low dose of Roundup that it gets.
In normal weather, with younger plants, Roundup works so well you can fudge on all that stuff and it still will kill all the grasses.
But this year - the plant might have gotten such a low actual dose into it's veins, it only weakened it, didn't kill it.