Yeah - and he mentions a timestamp in a youtube video - I was making fun of just that 
@JoSf I know that your math explanation was a hyperbole, but really, go check unigine docs, just so that youâll see how good the documentation actually can be. Notice the cleanliness, the ease of use, the tree structure, the topics like âprinciples of operationâ, check the physics section, which explains pretty much everything, check the âvideo tutorialsâ which contains their videos with some description, and, finally, note that they actually have ascection even on math 
Matrix Transformations - Documentation - Unigine Developer
If they would have Niagara, they would have like 20 pages about its core principles, typical usage, 30 bite sized examples and so on⌠(again, see their physics for reference).
To add to it, writing docs kinda forces you to think about the API, the end user, the full system interaction and so on, which in the UE feels (to me, the amateur, who cannot spend hundreds of hours to its source code) like a bunch of islands, rather than one solution.