With snapshots, while not perfect, they do catch a lot and are easy to do (and more importantly, easy to update when things change). React components are relatively easy to further test with enzyme and jest.
Without a focus on instructing people how to do testing, people never bother.
React, with it’s declarative approach, stateless components, immutable state, etc. seems purpose-built for testability. TDD with React should be ridiculously easy, but I wouldn’t know, because no one seems to want to talk about it.
The first examples are better off with just using snapshots. Even if the author is trying to explain the concepts, it is really poor to show bad examples that could be covered by a simpler solution. If I read that and didn't know about snapshots, I'd think to myself, "why am I testing strings?" and then skip testing entirely.
Mocking Axios also seems like the wrong way to approach things. I'd extract the networking code from the presentation code. Possibly by using mobx-state-tree (which is an epic solution to a hard problem). This way, I wouldn't even need to mock Axios at all and I could then just take advantage of snapshots again by providing actual mock data to my components directly.
I hope that more people building the learning courses will focus on that. If I had time, I'd do one myself, as I now heavily test all my React stuff. I refuse to build another webapp without proper automated unit testing. It catches so much stuff.