RMS wrote about free software and open source (which is different):
https://www.gnu.org/philosophy/open-source-misses-the-point....
https://www.gnu.org/philosophy/open-source-misses-the-point....
"Free software" is also an absolutely awful term because for 99.9% of the population "free" means it didn't cost them any money. This will never change no matter how hard RMS tries because its a very common and understood word.
> The terms “free software” and “open source” stand for almost the same range of programs.
and he definitely doesn't claim that open source is proprietary.