I think the first part of your argument is debatable, especially in modern times. In recent eras advancing industrialisation has meant telecoms, the internet, computers. All technologies that allow people to live and work anywhere. Working from home or working remotely is now becoming commonplace for the first time since the pre-industrial era.
Industrialization might have correlated with population movements from rural areas to the cities in the past. It's not so clear it still means that now. And as for falling fertility rates, that's surely more related to improvements in healthcare. You can say those improvements wouldn't have happened without industrialization, but that's just redefining industrialization as progress in general. In which case "cosmopolitanism is caused by progress" becomes the argument: an opinion, surely.
These cities are far more interconnected, both internally and externally, than the countryside.
You're talking about interconnectedness, the first thing the article's authors specified, but I was talking about internationalism, the second. The term "interconnectedness" is too vague to discuss; if it means people depending on each other, well, I've seen many claim over the years that cities are isolating and true communities where neighbours rely on each other are a suburban and rural phenomenon. I have no data either way. It seems easier to reason about internationalism, and that claim is dubious.
Also, I hate it when someone claims they have an alternative explanation, but don't present it so that we can examine it critically.
I didn't pick any specific explanation because there are so many to pick from.
For instance, if we drill into the definition of "populism", whereby it sets "ordinary people" against "elites", the most direct and simple explanation is that maybe in recent times there have been transfers of power away from ordinary people towards elites, and "populism" is simply politicians reacting to the concerns that this has caused. Obviously power being held by elites is a feature of many eras in history, has never been popular with the common man, and has nothing in particular to do with population growth or decline.