Since we don't know why Squidward wasn't working at the Krusty Krab on the day SpongeBob offered his Krabby Patty with jellyfish jelly to one of the customers nor what he was doing on that day, we can only make some guesses. One is that Squidward took a day off. Another is that he was on vacation. A third possibility is that Squidward was still in the hospital recovering after he ate multiple Krabby Patties in "Just One Bite", which, in fact, aired the same year as "Jellyfish Hunter".