This is one of the myths that feeds React's popularity. The few times I had the chance to be part of a project using an in-house framework, it was actually quite pleasant.
You can (and they will) build a massive house of cards on top of anything, in reality that new React dev will the take the same amount of time to ramp up, or worse if you're not careful. Learning the extensive collection of libraries in use, how they are glued together, application architecture, entry points, data and state management, build, deployment, asset management, etc is what takes most of that time. Having documentation for APIs can't help you there.
You’re still not taking into account resume driven development when they get ready for that next job. Would they rather say I built the website using Bob’s bespoke framework or React?
You can (and they will) build a massive house of cards on top of anything, in reality that new React dev will the take the same amount of time to ramp up, or worse if you're not careful. Learning the extensive collection of libraries in use, how they are glued together, application architecture, entry points, data and state management, build, deployment, asset management, etc is what takes most of that time. Having documentation for APIs can't help you there.