Game version affected: 2.10.149954

At this moment according to limitations described in Wiki:
https://community.bistudio.com/wiki/Arma_3:_Headless_Client

The server doesn't allow arbitrary connections from headless clients if you do not define the headless clients IPs in the server.cfg, the below value defines accepted IP Addresses from Headless Clients.
headlessClients[] = { "xxx.xxx.xxx.xxx", ... };

There is no way to set or bypass headlessClients[] setting for non-dedicated server because server.cfg is being ignored. The scenario when it is useful is playing singleplayer or (1-3 players) missions with huge amount of AIs without setting dedicated server up. Listenserver + HC on the same machine (localhost) allows to utilize more CPU cores / threads to process more AIs even if you play completely solo providing better QoL due higher FPS.