If he was picking butts (heh) every day for 8 hours a day, he’d have to pick a butt every 10 seconds. That seems like an implausible amount of butt picking.

If you watch the video, you'll see he shows a many dozens all in a small area, just within quick pan of the camera. He can probably pick up one per second for several minutes. That, or scoop them up in a huge bunch with a little bit of dirt, and sift them later.

It's not that implausible.

He even says he initially expected it would take a few years to reach one million.

Assumimg you pick them one by one. If you found a popular smoke spot I bet you could sweep up hundreds within minutes.