Ben actually didn't make millions writing here. :-)
When monetization first started you weren't allowed to have a call to action and put your posts behind the paywall. So he chose to not put his posts behind the paywall. Gaining email subscribers was more important to him.
His call to action at the end of each post grew his email list to over 300,000. That got him a massive book deal and launched his other business ventures.
I don't think any of his articles went behind the paywall until he quit actively writing. So now it's passive income, which is awesome. :-)
But the millions of views and claps he got when he was the #1 writer on Medium, weren't monetized. Medium was just a vehicle to get him started and make millions elsewhere.