I think the word "guarantees" is too strong. While the statement does serve as a premise (and more specifically, is an example/counterexample) it does not guarantee the conclusion. We have no idea what constitutes perfect knowledge of a language. Perhaps it is actually possible to become perfectly knowledgeable about a language without living in the time and place it was spoken. Just because this is what the "best" language students do so does not mean that it is the only way to gain perfect knowledge.