I don’t have any problem with the development of a mega horde but to suggest The Whisperers shepherded this horde and contained it? What’s for certain is Alpha did zero sherparding, her belly is proof.
shareHer followers did the herding. That cockroach is fat and lazy.
The Mega Horde is the storyline reason the communities don't strike back at the Whisperers.
That looks like the same spot Rick cleared of walkers before. The Whisperers must've re-sealed the road with one part movable so they could herd smaller groups in there and close it off behind them. And we saw how the noise made by twenty thousand trapped walkers attracts more who just tumble down the cliffs into the quarry. After years of collecting dead people Alpha's got quite the stash.
Say ... I wonder if that super horde Rick and his people saw from the hilltop after their final battle with Negan was the Whisperers first arriving in the area?