I think it was genuine. That’s not to take away from Gomez’s performance and win but watching the way Zarate fought even in the early stages, he didn’t look ‘right’ to me.
These sort of statements are the toughest excuse/mitigating factor to really verify. It's not usually as hard to look into if a fighter did have an injury beforehand, or if one occurred during the fight... or get enough accounts to ascertain they were a party animal that lacked dedication in a more general sense over the course of their career (though it will still inevitably be debatable as to how much it affected them at certain times), but for more short-term specific things like this it's a lot murkier. It doesn't help that a lot of the time it comes out after a loss, which doesn't necessarily make it bull****, but does show that the fighter either didn't feel it was enough to cause a likely hindrance, or had no choice but to go ahead and take the risk. In Zarate's case he wasn't some up and comer with little clout or fighter on the slide that couldn't afford to try and reschedule things. He was at the peak of his career with a lot of drawing power. I'd be more inclined to give him the benefit of the doubt than something like Azumah's Nelson's bull**** about still recovering from malaria.