I'm a huge React fanboy, but if you try and step through the React code with a debugger, we're kind of a far cry from the days of Backbone and even Angular. For the common mortal, it has to be treated as a black box. Fortunately, its fairly stable, so it's rare you have to care, but...
Yeah, the ability to step into Backbone code and back out of it into my own code was one of the things I liked about Backbone.
That said, React's mental model is so powerful that I've almost never needed to actually debug into it to see what's going on. If you've got an issue, tracing the dataflow in your own code is generally sufficient. So, I'm okay treating it as a black box.