Comment by HarHarVeryFunny

Yeah - he chose it as a pithy/catchy description of the issue, then immediately clarified what he meant by it.

> In other words, it is easy to fall into the trap of abstracting away the learning process — believing that you can simply stack arbitrary layers together and backprop will “magically make them work” on your data.

Then follows this with multiple clear examples of exactly what he is talking about.

The target audience was people building and training neural networks (such as his CS231n students), so I think it's safe to assume they knew what backprop and gradients are, especially since he made them code gradients by hand, which is what they were complaining about!