It's future-proofing basically.
The page is ancient https://www.haproxy.org/10g.html but they were doing 10Gbits on Core2Duo in '09 (sans SSL but back then AES acceleration wasn't very common either)
Usually the only reason to use many cores is SSL, but since OpenSSL 3.0 that totally collapses under load, even then you're forced to significantly lower the number of threads (or to downgrade to 1.1.1 that about any high traffic site does).
3.1-dev is slightly less worse but still far behind 1.1.1. They made it too dynamic, and certain symbols that were constants or macroes have become functions running over lists under a lock. We noticed the worst degradation in client mode where the performance was divided by 200 for 48 threads, making it literally unusable.