They released this tool a couple of years ago https://stripe.com/blog/announcing-mosql

In the post they wrote

"Here at Stripe, we use a number of different database technologies for both internal- and external-facing services. Over time, we've found ourselves with growing amounts of data in MongoDB that we would like to be able to analyze using SQL. [...] MoSQL does an initial import of your MongoDB collections into a PostgreSQL database, and then continues running, applying any changes to the MongoDB server in near-real-time to the PostgreSQL mirror".

So they could have been run that migration on MongoDB.