Um? The entire GAME validates it. Common sense validates it. The fact that everyone BELIEVES you're the last Jedi, but then coupled with the fact you're NOT the last Jedi proves it.
The statement "the last Jedi" implies FACT that there are no other Jedi lurking anywhere. The statement "the last known Jedi" implies that as far as the people you talk to are concerned, you're the last one. But that in reality, you might not be.