Actually what they would need to say is "Must use TGMO 0w16 or your new-car warranty will be void." No one has posted language such as that from either the owner's manual nor a warranty booklet.You're taking the MM Act too liberally.
If the manual says "Must use TGMO 0w16", then you have a point. But it says "Or equivalent" and lists 0w16 and API SN as requirements, it frees up Toyota, as there are many aftermarket equivalent API SN 0w16 one can use.
Just like the Germans, GM, etc saying oil has to meet their specification, that's why they have a oil certification program, so that aftermarket products can be certified and used to maintain warranty and be compliant to MM Act.