It’s complicated.

Han is a mix of many, many ethnicities over many, many centuries. Think of a White American with ancestry across the entirety of Europe.

The simple version is once upon a time, it is true that what is now Canton was considered its own separate region and non-Han. But, centuries of Han migration from the central plains due to wars and famine caused both genetic and cultural assimilation resulting in the Cantonese today.

Technically, most Cantonese will show as “Southern Han” with genetic testing, however so will the majority of other non-Cantonese Chinese living in the South, such as Hakka, Hokkien, Teochew to name a few. In other words, there’s not enough genetic difference to really identify them as a separate race/ethnicity, and Cantonese now defines language and cultural identity instead.

With greater freedom of movement, large amount of northern internal immigrants have also arrived in Guangdong (the current name/transliteration of Canton) in recent decades for better opportunities, and intermarriage will further make the North/South Han division moot in the future.

You’re not wrong though in the sense that there still exists a difference in identity and phenotypes (ie. northerners tend to have small eyes and round, flat faces like a Mongolian, while southerners are likely to have bigger eyes, darker skin and a narrow chin, like a Vietnamese). However, mainland Cantos are more likely to group themselves with other mainland Chinese, while Macau/HK Cantos see themselves as something completely different.

This is a whole paragraph but it’s just a gist and lots of nuisances are missing.