It's a bit of a circular definition. Certainly it shouldn't be surprised that the set of popular things is full of things that are popular. It shouldn't be a surprise that people are focused on fashionable topics since the notion of fashion is defined by what people are talking about.

The runaway effect though ensures that once was a small fashion becomes a stronger fashion until it overrepresents what people would otherwise naturally care about.

>Can you find any period in history when this is not true

Probably not.

> it just seems like restating a fairly obvious fact about what happens to human society

obvious and sad fact.

>as it grows.

why as it grows?

Maybe it doesn't even need to grow but Eternal September's are definitely a real thing.

When it comes to prescient-ness, reading some ancient Greek history (contemporaneously written) really changed my view in that if you read what (IIRC) Thucydides is writing about it's pretty much exactly the same as today ("An old man complained in the streets that the youths are spending too much time lounging around instead of fighting") but slower.