Apple presents it as a 3-tier fall back, where you may get the highest dynamic range that your setup supports, suggesting that HDR10 is available for all titles marked DV, in addition to the titles marked HDR. I agree that I wish for inclusion of both a DV symbol and a HDR10 symbol where applicable. I think HDR10 streams are available for all DV titles, but the exact wording in the documents allows room for exceptions. HDR10 is an open standard, so licensing costs are not an issue for including the option. The “down-conversion” for dynamic range already happened when preparing the files, so the servers send you what you need. No “down-conversion” for dynamic range happens on the media player for streaming content.
About 4K, HDR, and Dolby Vision on your Apple TV 4K - Apple Support
Find and watch movies with 4K, HDR, or Dolby Vision - Apple Support