First, activist language is pretty hard for any human to listen to. Anyone that's been tackling the "long fight" of what they view as oppression has this map in their head of things that add up to their main point in life and cannot help but continually extrapolate it for people. On some topics, people eat this kind of language up. On others, it's rather dull, especially if the person using the language can't quickly connect relatable points for people. RMS was good at long-form broad thinking that was dealing with the issues at the time. He was pretty fuck all when it came to really explaining them in a digestible, conversation style manner.
As for why proprietary headphone firmware hurts free software: take some Bose noise cancelling headphones and try to connect them to something like a mainstream Linux distribution like PopOS and use them on Zoom. They'll work, but only in a low quality mode, or the audio receiver will work but not the microphone. Now do the same with Airpods, or use Airpods on an Android phone. Airpods will work and you'll get high fidelity audio, but they're constantly exhibiting weird disconnection behaviors. On Linux you'll get incredibly low fidelity audio. Proprietary drivers are at the source of all of this.
RMS came from a day where the pioneers of Linux and alternative operating systems weren't building them for servers. They were building them for users because Windows was expensive and the machines to run it on were more expensive. Oppression to someone like him, and many others, were linked to this and many other circumstances at the time. The idea that the optimizations of computing were available only to a few was utterly disasterous.
To connect the two ideas, product x which is proprietary works best on product y which is also proprietary. It's a system that reinforces itself and excludes or marginalizes the rest, which eroded the original mission of building optimized software that is collectively free enough for anyone in the world to use.
Somewhat ironically, a lot of people in this thread are complaining about diversity when really that's what most of these initiatives were born in.