That's quite impressive. If you go by sheer numer in a city, Pittsburgh could have more because of Carnegie Mellon, affiliated institutes, Uber research center, and startups stemming from them.

The number of AI researchers in the Bay area might be even larger. Many researchers associated with large companies publish papers at a rate comparable to academics. Boston, Redmond, and Beijing are some other contenders.

Edit: Researchers in large company and institutions are not 'independent' so not included in their quote above. So yes, technically the article could be right, but it should not matter much whether the researchers work as part of an institution as long as their work is publicly published.