And two is what they call the cultural radicalism of the modern progressive movement, which they dub the “shadow party,” and that they argue is alienating working class voters on four key issues: race, immigration, transgender rights, and climate. (emphasis mine)

Their point isn't that it's wrong to want what the progressives want, merely that it's presently alienating working class voters and making it more difficult for Democrats as a party to be effective at getting any of their goals accomplished. Frankly, I agree with that assessment, and I'm pretty progressive. I think the problem is that we're advocating for too much change, too quickly. I know that sucks to hear, because in most cases we're talking about fundamental human rights, but I do think that's a major barrier we're running into.

Take gay and transgender rights for example. Back in the 80's/90's, the fearmongering cry around gay rights was that "this is just the beginning," and "soon they'll be saying you can marry your dog," and "this will destroy the nuclear family as we know," blah, blah, blah. Obviously, none of this is true, but to a conservative or conservative-leaning independent who is old enough to remember that time, when they hear a progressive today argue that "gender is just a social construct" and "it takes a village to raise a child" and "trans women are women," it really does sound like a certain amount of that old fearmongering was correct—maybe it didn't predict the future exactly, but close enough, this shit's crazy! And that contributes to the myth that there's a "gay agenda," because it makes it seem like there's a long-term plan at work that's being enacted gradually in order to sneak it's final, horrible form in under people's noses.

And that sucks, because I know plenty of people are sick of waiting to be treated the same as everybody else and morally speaking they shouldn't have to wait. But I increasingly see that it's not just conservatives that are opposed to some of what progressives are lobbying for. I know plenty of Democrat-voting Black and Hispanic people who are nonetheless (albeit quietly) opposed to gay and transgender rights, or who are suspicious of Asians or Arabs, due to COVID and 9/11. I know legal immigrants who are staunchly in support of Trump-style immigration policy, because they don't think it's right that people sneak into this country when they took the long, laborious, legal route. There was even a poll I read not long ago that indicated about 25% of Democrats aren't in favor of completely legal abortion (e.g. they're fine with it under certain circumstances, but don't think women should be allowed to use it as a form of birth control outside of those circumstances).

The article is saying that by catering to it's Far Left progressives, the Democratic party is hamstringing itself in terms of its effectiveness, and thus limiting its ability to get anything done, much less what the progressives want. It's a pragmatic argument, I think its correct. It sucks, but that's the way it is.