It's more nuanced because many things are never really found false, or can't realistically be falsified. Planck was implicitly referencing the whole controversy over quantum mechanics, and how exactly to interpret it. None other than Einstein refused, all the way to his death, to accept a probabilistic interpretation in spite of all evidence pointing in that direction.
He simply believed on a fundamental level that the universe must be deterministic. Many don't understand that many of his famous quotes like 'spooky action at a distance' or 'god doesn't play dice' were mocking probabilistic interpretations of quantum mechanics, like the Copenhagen Model - the one we still assume to this very day.
Of course there even remains the possibility that at some point in the future he may even be proven correct. And at this point I'm playing into the very same mindset that essentially explains where Planck's Principle comes from, but perhaps such being a part of humanity is not an entirely bad thing. It's certainly been with us even as mankind has gone through unprecedented leaps of understanding and technological advancement.