How do we know Splat Power = ELO

As far as I know, ELO only really works in a 2-player setting, and while Microsoft made TrueSkill to bring ELO to multiplayer games, it's also trademarked, patented, etc. so I highly doubt Ninty would be paying to use it in Splatoon. Are we just calling it ELO because it's some number that goes up and down depending on performance?