That's simple - it is provably wrong. While relatively uncommon there are plenty of examples that would contradict this statement. And it's not about being able to encode the Rosetta Stone - non-scientists mix languages all the time, from Carmina Burana to Blinkenlights. They even make meaningful portmanteau words and write them with characters from multiple unrelated writing systems, like "заshitано" (see - Latin and Cyrillic scripts in the same single word!)