It seems to me projects like that are great for quickly writing some math formulas, do some non-formal papers for school and so on. But I can't see how replacing LaTeX is even in the intended scope of the project nor how it can come close to it.
I think it's good enough for writing a journal paper. The journals would probably need to stay with LaTeX for the powerful typesetting and layout features, but converting from this MathML-Markdown mix to LaTeX should be trivial for them.