React is a view framework, and knowing the internals of React is a completely unnecessary set of knowledge for the majority of developers using it to build products with.
As with anything, the longer you use a library, the more you must know at least some of it's internals to use it efficiently/axiomatically/well. Even something small like the class/className attribute thing is an implementation detail that leaked through, which will bite and likely confuse a beginner for 3 seconds if they didn't throughly read the docs.
Other libraries don't have that problem (because they didn't accept that trade-off in that fashion).
IMO Libraries that beginners should be shown should not be ES2015-in-every-example, and introducing transpiled DSLs for generating DOM elements.
