Their email and even their “Plex: Free vs Paid” page is confusing. However, the “Requirements for Remote Playback of Personal Media“ is more clear.

I do not have a Plex Pass, but I stream remotely from a Plex Media Server

To stream video remotely from a Plex Media Server, you will need either a Remote Watch Pass or Plex Pass subscription on your account or the admin of the Plex Media Server from which you stream will need a Plex Pass subscription on their account.