Many different contract types out there. And I agree maybe he is not sponsored by adidas at this time.
I've had gear only contracts before where I received minimal clothing and/or shoes, but was not required to wear these. There were meager cash bonuses if I did wear these and they were clearly visible in media photos.
Maybe someone just gave him an adidas top, shorts and socks and he chose to wear it that day. Or that is what he bought.
Maybe he used to be sponsored by adidas, but is no longer, and the leftover adidas kit is the best clothing he has.
Maybe he used to be sponsored by adidas, is no longer, but wore out all his adidas shoes. Wore the Nikes in this race, but would like to be sponsored by adidas again so he tried to camo the Nikes. Or is fishing for either a new adidas or Nike contract so wore both.
It is possible his adidas contract expired at the end of 2018, and he has yet to resign.