I see no reason to doubt that, with all the changes going on with the ice sheet etc. But even in more stable times, civilizations flourished and then for reasons, fell. It doesn't mean everyone everywhere went backwards, right?
You are right in that it doesn't mean everyone everywhere went backwards.
It would be stupid and nonsensical to say that Australian Aborigines had gone backwards when they hadn't gone anywhere at all. The same could be said for North American tribal groups and sub-Saharan tribal groups who still hadn't figured out what a house was or the benefit of growing crops.
There was an advanced civilization in South America that did collapse for reasons unknown. Actually, there were about three advanced civilizations in South America that collapsed and then much, much later, there were 2x-3x as many that collapsed for reasons unknown and then you have the civilizations extant at the time the Spanish arrived.
Ancient civilizations in India/Pakistan collapsed for reasons unknown. Southeast Asia is the only region that appears to have continually advanced, although it can be said that Chinese civilization ultimately stagnated due to Confucianism and then collapsed politically, socially and economically, but not scientifically.
The Greek-firsters are very ethnocentric. Civilization to them is basically the eastern Mediterranean and Western Asia, much in the same way civilization for the Romans was anywhere Roman soldiers were, and that is the same view of religions that originated in Mesopotamia and the Levant.
Mesopotamia really did collapse and it was chaos for about 1,000 years.
Greece really was a hop-skip-and-a-jump backwards scientifically. We go from the Earth being a sphere and orbiting the Sun to Earth being flat and the center of the Universe.
If there was one thing you could do to change the world, it would be travel back in time and slay Aristotle's mother and father. If it wasn't for Aristotle, we would have landed a man on the Moon 500 to 1,000 years ago instead of 52 years ago.
That's the harm that Greek civilization caused Humankind.
Medically speaking, Mesopotamia was on a par with 19th Century Europe/America. That's the damage the Greeks did. No telling how many Millions died because of the Greeks.
I won't even get into the Greeks concocting the concept of Hell because of their flat-Earth nonsense or their concoction of a soul because they couldn't understand why people are conscious and self-aware.
That nonsense continues to have a negative impact socially, medically and scientifically to this day.