I'm not speaking for Keybase, but their approach so far is to fill these roles in compatible ways.
As an infrastructure provider, they open source the client software and design it to trust the server as little as possible. They also endeavor to document the behavior of the server so that, in theory, you could build your own Keybase-compatible server.
As an economic entity, they're positioned to tackle the issues that open source projects usually face, like support, development resources, UI design resources, and "where do you put all of this encrypted data" by getting their most needy users (mostly businesses) to pay them.