I don't get it.
Mjolnir is the name of the iconic armor master chief wears in Halo. That particular link is about a permutation of said armor that increases speed by utilizing "fast twitch" muscle fibers. Fast twitch muscle fibers are the stereotypically attributed reasoning for black excellence in physical displays. Terraformers is a manga that uses black stereotypes in a denigrating fashion.
Thus, the joke is explained.