Plex works even without DDNS and port forwarding. What you are talking about is otherwise not possible for people with restrictive firewalls or with CGNAT.
I agree with you that if you expose the service through the internet, then yes, it's just a DDNS remapping and their costs are tiny, but if you don't it does go through their infra.
No that's the thing. Plex can also use their infra as a tunneling system. You can have remote streaming without exposing Plex publicly and without VPN. It is slow though.