Check out this reply in the thread you linked: How to cancel FigJam billing? - #2 by Gleb
There is no “barebones free version”. Figma charges based on teams. If you add a FigJam file to your paid team, you get charged for FigJam. But you don’t get charged for files in Drafts or Starter teams, so you either need to create a starter team for your FigJam files (unlimited collaborators, up to 3 files) or keep all your FigJam files in Drafts (unlimited collaborators, unlimited files) without putting them in your paid team.