I don't know anything about the Sierra Club. I EXTRA don't know anything about their California chapter. Looking in to some of what you've said, I can see why they were a poor choice lol.
I just googled "mainstream environmentalist organization" and "housing policy" and clicked the the first one I found. Sorry if it seemed like I was pretending to know something about them - I was honestly just confused about what kind of evidence/organizations you wanted.
I found the statement "most environmentalists detest tall buildings" to be wildly cynical and unqualified, but I think what this is coming down to is that we have very different definitions of "environmentalists"(which I was hoping you would clarify).
To me (and to the dictionary, it seems), if you act/advocate in the interest of the environment, you are an environmentalist. Could be by protesting outside your local natural gas plant, but could also be by setting up special financing programs for CLT buildings, planting pollinator gardens in your front yard, limiting exclusionary zoning, etc. Given this - and given the fact that most of the environmentalists I know actively support building taller - it makes no sense to me to say that the majority of environmentalists are against tall buildings.
I get the feeling that your complaints lie with a very particular flavour of "environmentalist"*, which I don't think is at all representative of the true gamut of people who are doing actual, productive work on (vastly different) environmental issues. Suggesting that we need to "break the back of environmentalism to save the planet" is bogus doomerism... unless you are limiting your definition to something like:
"environmental organizations that oppose taller buildings in urban areas"
Which, yeah, I am not going to disagree with... because I am an environmentalist...
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*of the NIMBY, COEXIST-sticker-having, (probably) white-haired variety.