One might say Yang Kai's exceedingly low experience in handling day-to-day matters in sects was demonstrated here. Pretty much all of his time is spent on advancing his own cultivation or gathering resources for himself and his sect. In his own sect, he doesn't handle mundane things, he has got the managers for that. Now he suddenly needs to act as a referee in a situation where it grants him zero personal benefit, rather, it's wasting his valuable time. This is something he has never done before. Shopping for those seeds, however, was his own goal, so it's understandable he forgot about everything else, including his job as a referee, and made an elementary mistake.