Author here. I was greatly inspired by this website. It was my goal to actually combine metacademy's idea of solving the dependency problems of knowledge and do it in a similar way but visually.
I also try to ask help from other people, ideally experts in their fields on how to best structure the content. Mathematics one is quite challenging just because everything is really so connected as far as ideas go.
I hope as this projects gets popular, more people will be willing to help and improve it. It is a collaborative process after all.
I have a separate form on the site to get suggestions, but the email signup one has better quality because it weed out people who want to spam the site with their own content.
Here it is (bit.ly/learn-anything-letter).