A user seemingly representing the site you are linking to made some comments on the support letter's github page and they were not very welcome. The comments and responses are still there because nothing has been deleted.
Respectfully, you seem a bit confused. There are three such extensions that I know of. Some people on Twitter already talk about "no hire" lists. Not that I would would care to work for such people and their bullshitware. But it is clear that a lot of supporters are afraid to speak out, while I do not think the same holds at the other camp. I bet that some signatures in the attack letter are only there due to fear. Yet the votes in support are almost exactly twice as many and (important) it is translated in double the number of languages (30+).
Do not forget that the supporters are defending someone. While you may not agree about everything every single supporter says, this is all in response to the other campaign, which is attacking. The attack letter is such an incompetently written and fierce text that I am really baffled. Do they really believe the things they are writing? Did they just do it intentionally? I do not think a single explanation applies to all people related to that campaign. I have at least three in mind. In the end they and their obscene accusations and demands are backed by corporate money anyway. Livelihoods and careers are threatened. Why?
RMS actively supports all these groups that the letter claims he is against. Just look (besides his general character and actual actions) at his thousands of political notes, not just the three that are supposedly bad. He is not paid to write them. And it is not the same generic, colorless, tasteless, mechanically generated copypasta that is thrown around by people that are paid shitloads of money and claim to care about diversity.
Just look at the bigger picture.