Going by the public information on their website, the only way the company was making money from its product was via support and training.
The problem with making money only with training and support is that it falls flat when the product and documentation are so well done barely anyone considers these things important enough to spend money on them.
Additionally the website doesn't really provide much information on these services other than a contact form. If you're not already in touch with RethinkDB devs, you're probably more likely to ask a local contractor or specialised training company for help instead of reaching out to RethinkDB.
By all appearances, RethinkDB has been pretty successful as an open source project. It has a strong and active community, it is well represented at meetups, conferences and podcasts, it has 16k stars on GitHub (MongoDB has only 10k, MariaDB and MySQL have only 1k each).
But the company was not able to turn any of those numbers into revenue. Downloads, GitHub stars and Twitter mentions don't pay to keep the lights on. A successful open source project does not equal a successful commercial enterprise.