Freestanding rigs do exist, but as you're likely aware, they transfer all their loading and stresses through their mast step - most often being stepped through the deck so you essentially have the full hull height worth of support. Not sure which AC class you're referring to either - all the wing based boats absolutely have standing rigging, they just don't have spreaders since they're rotating masts.

Even if this proposed mast is stepped through the entire container, the forces on it will be plenty to cause issues since the containers are only held down by neighbouring containers and the 4 anchoring twistlocks. This proposed design will 100% require standing rigging to support it - the claim that the rig will only experience 10s of kn of loading is almost certainly incorrect as well - 30ft racing sailboats will experience loads exceeding that at the chainplates regularly. The moments about a mast are huge, even on relatively small rigs.