But somehow, you are allowed to imply that cleaning chemical reactors wasn't much fun, while at the same time disallowing other people from expressing negative feelings about their jobs.
By that logic, Y has no right to make this complaint if anyone who has worse job than Y exists. They do exist, therefore Y has no right to complain about X's complaining.
Y's position is inconsistent. It says "You have no right to complain because your job isn't hard enough, but I have a right to complain because my job is/was hard enough".
It establishes an arbitrary baseline of having-a-difficult-life and consequent implied manliness/human worth that Y is above, but X is below. It's a way to smugly look down on someone for not being as awesome as you are, while trying to project a humble "I did a shitty job" image.