Incorrect, they've never been about that.

Mao and most of the other early communist leadership were southerners. Most of the early KMT was made up of southerners too. I think westerners have this misconception that "Mandarin" is a northern language, but the standard Mandarin spoken in China is largely artificial, and has been a separate language for governance and education for centuries. No one in China back in 1949 would have thought of Standard Mandarin as representing "Northern" Chinese culture, even though it is based on an aristocratic dialect of Beijing Mandarin.

Some of the most endangered languages in China are natural forms of Mandarin in northern China, because it so similar to Standard Chinese and the government has no protections or cultural programs for them, unlike the southern languages like Wu, Minnan, and Cantonese.

China is a complicated country and it's sad to see these misconceptions repeated ad nauseum in English media.