Being "well diversified" is subjective. But in freelance/contracting, the rule of thumb is to never have a customer account for more than 25% of your yearly income. You diversify your client base so when one walks away, it's never too big of a hit.
Ads make up 80% of their revenue. That's not diversified. That's an ad company that dabbles in other things. If Shell or BP's revenue was 80% oil and 20% solar, wouldn't you still call them oil companies that dabble in solar?