There must be thousands of people with sites monetized by things like AdSense, whose sole traffic source is SEO (for which domain-names play a significant role), who would disagree rather strongly with this statement?!
Domain name actually plays a negative role in organic Google search rankings since the exact match domain algorithm update in 2012. There's no one out there in 2016 who puts together a random site, does some magic SEO, and gets people to both visit the site and click on ads. You have to put together a site with real value to users to get and maintain high organic search ranking, and the site has to have compelling content to get them to stay long enough to click an ad. It really needs to get them to return over and over or you need to get literally millions of users since you're making so little per click. Again, a domain name gets you exactly 0% of the way on that journey.
Sorry, but this simply isn't true: what you're thinking of, regarding the SEO-penalty, applies only to exact-match domains specifically designed to fool Google's algorithms, as very openly explained on Matt Cutts' Google blog, in articles by him, and on Google's own help pages. Domain names are still hugely relevant to SEO.
How much SEO have you done? Have you ever put together a site in a field as crowded as this one and ended up above the fold on organic search for any term? Or even in the top 50 for that matter? Do you seriously think that you're going to get $5000 worth of SEO optimization out of the miniscule move that the domain name is going to have on SEO rank (which could very well be negative)? Do you realize how much AdSense ads pay out, what the click rate is, and how much traffic you'd have to drive to your site, as well as the quality of that traffic, to make up for $5,000 you paid for that domain name. In no business case anywhere will that pencil out for "No1Trader.com" or anything like it, and certainly not when compared to the many very similar domain names that others pointed out are available for simply the registration fees. Sorry, but if you think it's that easy than you've clearly never done it!
I'm going to say he's either BSing you or oversimplifying how he makes money. Do you have any of the URLs that he's making millions off of? I'm willing to bet he either sold them to companies who are actually selling things or producing content on those sites (back when they thought domain names were valuable), or he's selling things and producing content on those sites. In either case the profit came from selling things or producing profits, not the domain name. Again, would love to see an URL?
You haven't even read the thread, have you? I was among the first, on the first page, to point out that "thousands" for this domain is living in a dreamworld. (I valued it at about $100-$200, max.) Time for me to unsubscribe from all this argumentative nonsense.
Good example even if he didn't own that specific one. Back in 1998, no-one really knew or understood how the internet worked and we didn't have good search engines people would actually randomly type "pets.com" into their browser if they wanted to find out something about pets. Company's had mad VC money they were throwing at what later turned out to be positively horrible ideas and the few people who had snapped up domain names were able to sell them to the pets.com's of the world for crazy amounts. Flash forward two years to 2000 and pets.com is bankrupt. Flash forward 20 years to today and pets.com is a site making money because of the brand and fact they sell pet supplies and while their domain is part of their brand it's not what they earn their revenue from. I can tell you that No1Pets.com is worth exactly nothing on it's own, nor any other variation on pets.com. It's not 1999 any more, domains that haven't already had a brand built around them are on their own worthless and everyone knows it. The ultimate irony is the the most recognizable and arguably valuable URL in the world is a made-up word derived from the word for 10^100, or as we know it today, google.com.