Like everything, it's a balance -- open source can also be used to make money (open core, closed source plugins/enhancement suites). At the very least, it's about tact and doing things that are just open enough to not betray your desire to create a walled garden.

Let's use Google's Kubernetes as an example -- it's a fantastic tool that doesn't require use of their ecosystem -- so open in fact that both Azure and AWS are spinning up their own hosted versions. Many of the architectural/organizational pieces are non-trivial as well, years of work at google went into it, and they open sourced it.