I'm going to generally agree with most of this while picking out one pretty nitpicky point that I want to counterpoint.
A lack of effective relationship and solidarity building across class, color, gender, religious beliefs, sexuality, age, ability, culture and more is part of the problem.
A lack of effective relationship and solidarity building across economic status, and across "goodness" of the belief of the person being solidaritied with, is the lion's share of the problem.
The modern progressive left defines itself chiefly in terms of equality of color, gender, religious belief, sexuality, ability, and so on. Most people in the United States, I am sad to say, do not feature that as their primary concern if they give a shit about it at all. Most people want to know that their individual life is going to be better, because it is hard right now, because someone is stealing all of the money they worked for. You could say it would be a better world if they also wanted life to be better for people of different colors, genders, sexualities, and so on, but that's not the world or the people we've got, and they're not wrong to chiefly worry about the "rich people are stealing everything I'm working for" problem. We can help any grouping and gather support from any grouping, without needing to say we need to put the brakes on and help some other more virtuous-to-advocate-for grouping instead.
Building economic success for the average working person, whether they be black, white, straight, gay, whatever, should be in the wheelhouse for the American left. It should be a slam dunk. Every time Bernie Sanders or AOC starts talking about it, people start cheering so loud they shake the fucking building. But, a lot of what the progressive left wants to talk about is all of the progressive left's favorite issues. Meaning equality of gender et cetera.
Health care.
Jobs for working people. Unions.
Real democracy instead of management consultants and incessant bullshit text messages asking for money.
That is my one point of input to what otherwise I really like and agree with as a post, and maybe it seems nitpicky and bitter. But I just want to throw in that it would be a good idea, when building all these coalitions that the article pretty exhaustively goes over good guidelines for how to build, not to lean too hard on the purity test that someone needs to be able to use pronouns to your satisfaction before you are willing to collaborate with them on both of you not having to skip meals, or get sent to death camps for unionizing or being Hispanic, or what have you.