Let's have this example,

print(JSON.stringify(JSON.parse("[1, 2]", function (name, value, context) {
    if (name === "0")
        this[1] = [ "Hello" ];
    return this[name];
})));

JSON.stringify's result is [1,["Hello",["Hello"]]] before this proposal because reviver modified array[1] at runtime and we visit this newly inserted ["Hello"] array.
But for this array, we do not have correct corresponding parse information since this is not coming from text. source information for this property is still 2's information.
Thus,

i. Assert: typedValNode is an ArrayLiteral.

This will crash since this newly inserted array does not have right source information. Source information for this property says 2 (it is coming from the original source).