In reply to Gary Taylor.

I chatted with the Themes Team about what was going on there. It was the theme authors who hadn't added the full-site-editing tag. They've been asked to in their respective Trac tickets. Q, at least at last check, has the tag in its GitHub repo but hasn't been updated on .ORG yet.

Thanks for bringing this up though. Before your comment, I hadn't realized the filter wasn't showing any themes.