The language barriers makes Europe much more culturally diverse, but also makes that diversity much harder to discover since you can't fully understand a culture without learning its language.
It is more like the cultural difference between Canada and Mexico, you don't have such a wide spread within USA. You have some parts that is more like Canadians, and some parts more like Mexicans, but Mexico to Canada is still a bigger cultural leap than anything within USA.
The opposite is true in the USA: nearly everyone speaking English to each other, watching the same TV shows, driving the same cars, eating the same breakfast cereals - easy to believe they must all be culturally very similar, when in fact they are not.
But we also know that there are much bigger differences if you cross many national borders and language barriers by going to the other side of Europe, they too will have those micro differences but when going there you also get many major macro differences.
Decades later, that's clearly an exaggeration or just plain wrong, but it is certainly true that people do underestimate cultural differences within the US, and overestimate them within Europe.