That is an awesome question. Yes, it is an insult. He is your boss, and is throwing a party to have people show up. If everybody flaked like Sick Gambler, how would the boss feel. The slang term is ticket punching. It is a time to prove yourself away from the workplace. Actually an opportunity. There will be the drunks, the fools, clowns and baffoons. Not to be confused with balloons. But they do deflate, or let the air out when making fools of themselves.
My advice, show up early, help set it up. Say you have a later pressing engagement, but wanted to contribute. You comes across as considerate and thoughtful. The boss will not forget that. As soon as someone starts acting foolish, the time to leave has been offered. You will look dignified, while other person looks like a party pooper, acting the fool, drawing the scorn of others, especially the boss. Do not get drunk. Do not be rude. Make the rounds, be the social butterfly then bail. You punched the ticket, shmoozed the boss and left with dignity. Expect the raise soon after.
Best Wsihes...oF:+waving-5