Stripe runs a ton of Mongo replication clusters and uses home-grown proxy services on top of Mongo that manage and control where data lives, so the services don't have to think about that side of things. I'm not sure what changes have been made to Mongo itself but for the most part it's standard Mongo 4.

If you take open source MongoDB and add 10 lines of custom proprietary code, is it still MongoDB? What about a hundred, or a thousand, or a million? What if you change the interface? Ultimately you can give the modified service any name you want, and whether it is "actually MongoDB" or not is a subjective judgement.