You can say the license isn't "open source." The term has a well-defined meaning provided by the OSI, and they arguably have the right to define what it means and which licenses meet the definition, being the ones who pretty much invented the term.
You DO NOT, however, get to also define the meaning of the word "proprietary." The English language is not your plaything, and you have not been given dictatorial rights to re-define words as you wish. "Proprietary" does not suddenly mean "restricts any of the four freedoms" just because you said so. When antt calls this instance a misuse of the word, [s]he is relying on the common English meaning of the term, which very much supports their point. Your rebuttal is pretty much "nuh-uh because we're now using a different definition."