That's correct. ODR content is more likely to be retained than data stored in temp or cache directories, and you can manage your own assets for ODR in terms of purge priority.
If you have content that is required for initial app launch, you can include up to 2GB of tags as Initial Install, which means the assets are included in the initial download from the App Store before the app can be launched. You can also mark assets for "prefetch", which means that the app can be launched even if they haven't been downloaded, but that tvOS will automatically start downloading them in the background when the app is purchased.
See this page on Platform Sizes for On-Demand Resources for more detail on the various size restrictions.