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The original data includes "train and metro stations", but figure 9 filtered the data to only include train stations and arrived at the same conclusion.

At the end of article it's shown that only considering train stations didn't really change the result.

That saying in France is usually understood to be for cities outside of Paris and only referring to "Gares" (that word is used for train stations, not for subway stations). Anecdotally, I'd say it holds true in general in most cities I've visited (with Paris being an exception)