Configure HAProxy on Linux to accept 10K simultaneous connections
As far as I know, listen block maxconn is different then global maxconn. With global maxconn you limit the max number of connections you let the haproxy process handle.
Listen / frontend section has its own maxconn, which limits the nubmer of connections per listener. So, try to set up maxconn in your frontend sections too, or at least set it up in default section.
So either:
defaults
maxconn 10000
or set it up per frontend.