What exactly is the distinction between this and mathjax? At first glance it looks like this is a wrapper on top of mathjax, or is it something different?

I guess it’s just more “natural” and less latex like.

$$\sum_{i=0}^n i^2 = \frac{(n)}{2}$$

Vs

sum_(i=o)^n i^2=((n)/2)

asciimath does a lot of things automatically, like parenthesis sizing and auto-frac, that latex requires you to do yourself. \left and \right and \frac do add quite a bit of noise to simple equations.