While I've never seen anything explicitly say it, it's heavily implied from their earning and revenue sources - eg; ad revenue makes up a big chunk of their income, theoretically, as they aren't fully transparent with this but have read reports of them earning up to ¢50 a visit by player to a McDonalds sponsored location, for example.

They can then use the location data gained from a huge player-base to then advertise mutually beneficial contracts to companies such as McD, Starbucks, etc - I don't think Niantic are selling data specifically, but I do think they are farming it in order to make the most business-minded decisions when it comes to ways that they do earn money - and besides, they are a Google subsidiary, that data is almost definitely being analysed in ways we cannot infer. (is my opinion, at least.)

https://entrepreneur-360.com/how-does-pokemon-go-make-money-27912#:~:text=Pok%C3%A9mon%20GO%20has%20a%20revenue,game%20advertising%2C%20and%20selling%20merchandise.

https://insights.dice.com/2017/06/02/pokemon-go-money-sponsor/