It is annoying that the answer is basically "we paid someone [Cloudflare] to do it." In light of that I decided to run some of my own numbers and figure it out.
They are pushing about 245 Mbps out of Cloudflare (averaged over the month). Wholesale IP transit prices are anywhere from 10 cents to a dollar per Mbps depending on volume. Cloudflare dumps 40% of their traffic over peering, putting their price at $14.70/mo. Ignoring fixed capex of servers, Cloudflare is making about $25/mo on this customer.
Given the $11 Backblaze bill, I estimate about 2 TB of data.
A capable dedicated server with 2 TB of disk and 1 Gbps unlimited bandwidth will run about $30 in a major European metro, maybe double that for the US.
With a grand total of $370 vs. $60 worst case, they are spending 516% more to be "serverless."
Edit: Yes Cloudflare has more than one server. Double the price and put one in the US and you still come out ahead.
Edit 2: I'm not saying one way or the other is better. Just that the title is very clickbaity for a "put a credit card into a website" payoff.