Thank you!
That’s something we need to clarify with the Discourse team.
The Theme Component version is being developed to explore the potential to support the emerging extensions to the core API that David has been discussing here.
It is up to Discourse where these come from and how they are handled, as a Theme Component contains no API.
This new extension to the API exposes thumbnails. Historically these were generated by this Plugin. I will be very happy to retire that logic if at all possible, though there may still be work Discourse will need to do to reach the full level of functionality I’d need it to have to do that.