I don't think "cloning D&D" is what any of these third parties are doing. It's producing add-ons, source books, or videos of them playing. That is, original content that is compatible with, and tuned for the rule system.
Some are to an extent. There are several modern adaptations of the old 'Moldvay' Basic/Expert Dungeon and Dragons rules (The old red and blue books/boxes). Basically getting them down in a better way on paper, smoothing some rough edges out.
