This is more a problem with science reporting than with the science itself.
Saying 'modern humans' or 'homo sapiens' are X years old is a misleading statement. Evolution isn't some binary where all of a sudden you have modern humans. It's genetic change over time.
Better reporting would be to compare this find with relatively modern features in Morocco at 300,00 years ago, to Homo Naledi with archaic features in South Africa at 335,000 to 236,000 years ago. As other users put it, compare this data point to other data points.
This find implies that relatively modern humans had spread to multiple parts of Africa by 300,000 ya, while at the same time evolutionary cousins like H. naledi persisted in other parts.
"And if that is true, Dr. Gunz and his colleagues argue, then our species may have been evolving as a network of groups spread across the continent." This IMO is the best sentence summarizing the impact of this find, and makes the most sense evolutionarily. Instead of having one group evolve all modern traits, and then 70,000 years ago explode across Africa and then the globe, you have several groups with limited interbreeding evolving 'modern' traits. The interbreeding allows the traits to cross into other groups, and modern humans are the result of a particularly successful combination of traits where some traits arose in one group or location and some in others.
So sure, in evolutionary deep time (1-2 million years) East Africa remains the cradle of humanity. But this find updates the story that by 300,000 years ago, the groups that became human had left the cradle. That's a big story, and is a much better summation than the misleading "Humans are X years old, older than Y that we previously thought".