It doesn't have to be a lie. As far as I can tell they don't call them "customers" at any point, so they're most likely just "companies using RethinkDB". They

It doesn't have to be a lie. As far as I can tell they don't call them "customers" at any point, so they're most likely just "companies using RethinkDB". They might even use RethinkDB extensively -- that doesn't mean RethinkDB makes any money from it. It's not lying, not even misleading. It's just not a metric that translates into "this company is making lots of money". If you're a hugely successful open source project and every Fortune 500 company uses your tech that still doesn't mean you're financially viable unless those companies also give you enough money (e.g. for support and training, or proprietary licensing deals) to sustain yourself.