So reading the article you’re right, it’s technically anycast. But only at the /24 level to work around BGP limitations. An individual /32 has a specific datacenter (so basically unicast). In a hypothetical world where BGP could route /32s it wouldn’t be anycast.
I wasn’t precise, but what I meant was more akin to a single IP shared by multiple datacenters in different regions (from a BGP perspective), which I don’t think Cloudflare has. This is general parallel of ingress unicast as well, a single IP that can be routed to multiple destinations (even if on the BGP level, the entire aggregate is anycast).
It would also not explain the OP, because they are seeing the same source IP, but from many (presumably) different source locations whereas with the Cloudflare scheme each location would have a different source IP.
To be clear, they definitely use ingress anycast (ie anycast on external traffic coming into Cloudflare). The main question was whether they (meaningfully) used egress anycast (multiple Cloudflare servers in different regions using the same IP to make requests out to the internet).
Since you mentioned DDOS, I’m assuming you are talking about ingress anycast?
It doesn't really matter if they're doing that for this purpose, though. Cloudflare (or any other AS) has no fine control of where your packets to their anycast IPs will actually go. A given server's response packets will only go to one of their PoPs. It's just that which one will depend on server location and network configuration (and could change at any time). Even if multiple of their PoPs tried to fetch forward from the same server, all but one would be unable to maintain a TCP connection without tunneling shenanigans.
Tunneling shenanigans are fine for ACKs, but it's inefficient and therefore pretty unlikely that they are doing this for ingress object traffic.
