Thats the first thing it says. There are other interpretations in the article.

Stibel believes that a changing climate, and not language, could explain our smaller brains. In a 2023 study, he analysed the skulls of 298 Homo sapiens over the past 50,000 years. He found that human brains have been shrinking for about the last 17,000 years or so – since the end of the last ice age. When he carefully examined the climate record, he found that decreasing brain sizes correlated with periods of climate warming.