Back in the day it wasn't as easy to generate Bitcoin wallet addresses so people used websites that would help. If you Google "bitcoin paper wallet generator" you can find a ton of them. Simpler times when Bitcoin was still in its infancy and not worth a whole lot.
Anywho, this guy had used a particular site way back when. At some point, the original owner sold the site to a bad actor. Wallets generated on the site likely were captured by the owner and any BTC sent to those generated wallets were swept.