I’m also surprised that traffic from Cloudflare DNS users caused any significant problem. Was it really that much traffic?
Anycast IP is very expensive, unfortunately. Just getting a /22 has been expensive for years, and is now also getting difficult as well. It is beyond the reach of smaller companies.
GeoDNS is extremely cheap in comparison. You can run distributed services using GeoDNS for low latency on multiple continents on a hobby budget these days.
Anycast is technically better in many ways (the combination of anycast and geoDNS is better again), but anycast is so expensive that smaller operators just can't use it.
These days, smaller operators can use Cloudflare for their CDN, and the suspicious mind might think that suits Cloudfare just fine. But that doesn't really help for low-latency interactive services, or non-HTTP services.
> I’m also surprised that traffic from Cloudflare DNS users caused any significant problem.
Maybe the problem isn't amount of traffic, but rather that the site doesn't want to gain a reputation as slow (and therefore incompetently administered, and offputting to use) when everyone running Firefox switches over to 1.1.1.1 DoH automatically.