You don't need to guess. You can actually read the code! Not saying that you should straight away when learning, but later, when you're familiar with your framework, I think reading through the codebase and understanding it is good practice.
You can read the code, but it’s complex and convoluted.
He referenced Backbone as a comparison point. If you compare Backbone’s source code to React’s source code, you’ll get what he means.
Basically, libraries like Backbone are small and simple enough that you can literally read the source code and fully understand how it works. Compare that to React, where the source code is an order of magnitude larger and very difficult to fully grasp the inner workings.
The simplicity and ability to easily understand the source code obviously comes with tradeoffs (e.g. with Backbone, to use it to build a reasonably complex app, you basically have to build your own framework on top of it, compared to React which has more abstractions and therefore is more plug and play.
