It's something worth thinking about, because it's trivially true one could be a socialist and socially conservative, the same way someone can be "socially liberal and fiscally conservative," and so on. More precisely, it's trivally true one can believe anything.

What the author is leaving out (and this is what makes it just a complaint, rather than analysis) is why the coalition formed. Instead, he just says "The mystery is why," and hand-waves it as "whatever the reason..." But it's the reason that's central to the issue. This didn't happen out of nowhere.

Some of the reasons are just arbitrary. It is just coalition arithmetic. There's an argument that it was racial politics that flipped the parties (cf. Carmine and Stimson, Issue Evolution (1989)). It's disputed, but it's less ideological than it is political calculus (at least in America). People want to win elections. Kennedy moved cautiously on civil rights. And then eventually, Nixon looked at a map in 1972 and turned it into strategy. Nobody sat down and reasoned from Marxism to racial equality, it's the side-effect of a two party system. You can say the same for why there are a lot of conservatives who are both fiscally and socially conservative, so much that being "fiscally conservative and socially liberal" became a meme and clarification.

The other reason is de-industrialization. In the 50s, if you were young and left-wing, you go to your union hall. The labor movement was a huge part of the ground operations for the Democratic Party. Fast forward to today. The same person would probably be going to college, in large part because manufacturing employemnt has been dropping for decades. The factory is not culturally liberal. That's the professional culture. The campus is culturally liberal. That's the professional culture. At the end of the day, we work, and these are real social forces that move people in certain directions.

That said, it's not entirely arbitrary. Robin Kelley wrote this book, Hammer and Hoe, and in it he talks about black communist organizing during the Depression. The gist is that the Communist Party thought it was just going to educate industrial workers on straightforward class politics, but what they got instead was black workers taking over and bringing their own institutions with them: the black church, spirituals as organizing songs, people reading Lenin and scripture in the same evening, and so on. The author of the FT article is being pretty narrowminded or is just simply ignorant, to talk about "moderate socialism" and ignore the history of black communism, which is as significant as it gets, at least in the United States.

So the fusion -- even if it's not necessary nor sufficient for left-wing politics -- makes sense, and extends beyond race. We're looking at a social phenomenon where groups of people are left out, either by not having money or being the wrong kind of person. And I think when we're talking about a "culturally moderate socialism", it's inevitably going to invite the question "Who gets to count?" I don't have any theories on whether this is 'good' or bad for socialism, and I'm not interested in debating pros and cons of leftist versus neoliberal theories, but that's where it comes from, and I think the author should at least bother to think about it before dismissing it as a "mystery".