Souji was sure that wasn't a joke, because King could never keep a straight face for thirty seconds. On top of that, while Souji doesn't recognize the name at first, the context that is given makes his stomach sink and all he can think about is Ucchii and he knows this isn't a joke, because King would die rather than do something like this to anyone, let alone Ucchii.
He'd been laying in bed, sleeping in late. Thanks to his injuries, Souji had been unable to (and forbidden from) training and taking a risk at hurting himself more. Checking the journal was something he did every morning before wrapping it in fabric and shoving it into a drawer in the hopes it wouldn't accidentally record him. So, once it's haphazardly shoved back into the drawer, Souji is up and immediately out into the middle area of the base, his heart beating fast in a mild panic. He hasn't even bothered to change out of his nightclothes.]