Progressives don't necessarily agree with other progressives. At least a subgroup wouldn't self describe as such.
This was tried to communicate very often, for example with reference to diversity of opinion. It was, perhaps with some reason, seen as an argument against diversity of skin colors.
Which is fine, I guess. I personally didn't think the letter was so awful. It's hardly the worst problem in a world where we have federal paramilitary units being deployed to pick fights with hippies, but there are excesses (David Shor for sure shouldn't have been fired).
The problem is there's a baby vs. bathwater issue with the reasoning. The same people who spit bile about Shor are the ones who just pushed BLM from a fringe idea that couldn't get purchase into something approaching social consensus. Did anyone see the ballgame last night? What's your position on Kaepernick now?
The text of the letter is hard to argue against. The context in which it was presented, and especially the way it was leveraged on the right as an "a-ha!" moment to disparage many of the demographics that were supposed to have "signed" it was quite different.
Republicans view that letter as an admission of guilt on the part of the left, when the intent was to call back absolutist rhetoric everywhere. It didn't work.
[1] From the perspective of the activist left: the powerful looking to suppress checks on their power from new voices.
Please give us the decoder, because "it's code for whatever I feel like it is" is really not helpful.
Girard seemed to think that the great cultural problem of the world was "The Court of Versailles" where nobles who have no real problems just compete to be the same as each other. It's a compelling problem if you're a vendor who makes fancy stuff for the palace (e.g. one of those mirror makers who got assassinated to protect the secret of making mirrors) but for the 99% of people who grow rice, wheat, corn whatever it is that supports the life of most people and the vendors who serve the palace, it is just designed to erase your perception of your own life and make it a pale shadow of someone else's narcissism.
That's hardly useful, because it's more mysticism, and there's no testing your opinion, and so it also can't be rejected .