2 out of 5 of his clients are blockchain companies/orgs. I feel this path would be quite easy in blockchain, where almost all of the projects are open source
Also note that for blockchain projects, it's important that they associate themselves with famous people to give their projects credibility. So they are willing to pay huge amounts of money to get celebrities like him on board.
If your open source project has a homepage, a fair amount of visitors, and a public list of sponsors, blockchain companies will pay for the highest tier to be on top of that list instead of going down the usual AdSense route.
From their perspective it's a link from a respectable source that reaches their target audience (those into tech) on a permanent basis that even adblocks don't block. And it only costs them up to a couple of hundreds of bucks per month, way cheaper then traditional banner ads. Doesn't matter if what you're actually building has anything to with cryptocurrencies, but of course having some touching ground works even better.