Yes, however using the relay is not a prerequisite to being required to pay for a Plex subscription. That is what he is trying to say.

I can run Plex on the open internet and not use their relay at all, however if the IP of the viewer is not an interal IP on the same subnet as Plex (I assume the same subnet is required) then you’ll be greeted with the Plex paywall.

You are absolutely correct that it costs money to run a relay, but the relay has nothing to directly do with the paywall.