That code is for round wire (minimal surface area per volume) that can be placed inside insulation in walls. This 350A is flat conductors (maximal surface area

That code is for round wire (minimal surface area per volume) that can be placed inside insulation in walls. This 350A is flat conductors (maximal surface area thus heat dissipation) and very short (not that much power to dissipate so the things it connects to have a significant effect on heat dissipation).