A DoS is usually trivial to stop. Now a DDoS, that would be impressive. I don't need haproxy to stop a specific request or from a specific ip. The kernel routing tables can do that.
Yes, for a known IP address you can use the kernel. HAProxy ACLs can match user agents, URLs, referrers, etc.
I don't see anything in this article about DoS, where are you getting that from? these are benchmarks against static files, still very impressive but not DoS protection.
The section titled "Session setup/teardown rates" talks about using ACLs to block or redirect requests.