It's licensed under the Affero GPLv3, but I don't think AGPL license requires you to open source your code, if you're just using ReQL (the API/interface for RethinkDB).
GPL/AGPL requires you to share your code if you statically (or dynamically) link (which is a C language family concept) to GPL code, but I don't know if calling an API is considered "linking".
I don't think most companies really make any contributions or modifications to the databases they use that they absolutely do not want to share back to the community.
(If using ReQL constitutes "linking" under AGPLv3, then that's a very serious matter. Perhaps re-licensing under LGPLv3 would make sense then.)
But overall, I think it's fair that they used AGPL. I especially like that they opted for the Affero version, since companies that do make useful changes can't just hold onto it , and not share it back.