Not quite unambiguous! It declares how to parse numbers, but of course the "reference implementation" of JSON (insofar as there is one) is Javascript, in which numbers are floats and hence e.g. are bounded in precision, so the spec actually disagrees with the reference implementation.
I don't think this is true. The original specification (ECMA 404) only covers the formal language, i.e. what are valid documents and not what their memory representation after deserialisation is. A parser that rejects a number because it has too many digits would not be conforming.
rfc8259 is more detailed and it mentions that most parsers will have limited precision and references ieee754 to explain what the expected precision after deserialisation should be for most parsers.
