I wondered the same thing. "No, you can't mix data types, that's stupid" leaves it ambiguous.
If you parse the outer array as just "array of arrays" (as each element is an array), you're not "mixing". But if we're supposed to be parsing it as "arrays of arrays of _type_", then we are mixing.
If you parse the outer array as just "array of arrays" (as each element is an array), you're not "mixing". But if we're supposed to be parsing it as "arrays of arrays of _type_", then we are mixing.