I’d interpret this to mean keeping you API specification basic to minimise backwards compatibility problems and reduce future refactoring difficulty. Don’t have special methods for unique cases, stick to restful principles where possible, keep data structures simple, etc.

I'd add give enough freedom for users to accomplish what they need from a small set of API abstractions.

Many times, this shifts burden to users. It's always more convenient to have one API which does exactly what you need.

But the idea here is to keep the API simple and stable in a phase where instability is already high.

Perhaps, after it gets mature, it'll make sense to incorporate common use cases into the API structure for convenience.