iOS file type registration is really wonky. Drafts is the creator of those files, and registers as an owner of those files, but iOS does not always offer it as an option to open them for many users. It’s super frustrating.
I need to workaround this with an in-app restore option. That’s been on my list for a while, but is not in the app yet. I know many other apps that allow opening of files will work - for example if you move the file to Dropbox, open it in the Dropbox app and share, the “Copy to Drafts” option will be there and allow you to run the import.
Import support will be in the next Mac build as well.