According to the English language article https://www.zdnet.com/article/free-software-advocate-richard...,

'According to Segala, Stallman requests no video feeds in "places using proprietary JS."'

Therefore, we see that his concern isn't about the language of implementation, but whether it's proprietary or not. So you could write a FOSS web video player and post a video of him there, but he doesn't want to be on YouTube since I can't say "well, this video player would be much nicer if..." and then do that.