That does not sound right to me. “20 percent of websites” does not mean “20 percent of traffic.”.
There is no public write-up from Cloudflare that proves “we handle 20% of all Internet traffic.” Cloudflare reports around 295,000 paying customers and more than 30 million Internet properties (20% of the web). So most of their users are on the free plan.
I can't believe they only have 295,000 paying customer, that puts me in a small minority. lol