Because you’re only ‘exposing’ the port on the peer to peer network.
You “publish” a port to holesail, then clients have to create a local proxy via holesail before they can access it.
I agree, It’s a dumb pointless claim. But I don’t think it’s misleading.
It looks like holesail is just tailscale, but on a much smaller scale. It’s not networks, it’s just ports.
I doubt it’s /just/ smoke ventilation. Sure, it’s primarily there for fire safety.
But it probably provides ambient ventilation for the tunnel too. Tunnels with trains get hot. Most things, living or inanimate don’t like heat.
And if it’s always ventilating, it’s also might be expelling brake and metal dust from passing trains.
But all speculation. I’m not a train engineer, so I’m probably wrong.