Boise Ed sent this one in. If Hagar and Eddie joined the onstage table, they must have thought they were at a dinner theater. But if it were a dinner theater, the audience wouldn’t be in rows of theater seats. It looks like they went to a stage play, got there late, saw the scene involving a meal, then sat there. Even those two aren’t that dumb.
Comics I Don't Understand 
The gag isn’t about them being dumb, it’s about them being hungry. After all, they are Vikings: we should expect them to be rude, crude, and socially inacceptable.
Probably the stage arts had not yet advanced to the point of using prop food for meal scenes. So Hagar and company can expect to get fed for their “mistake” .
Wasn’t there a famous improvised comedy bit once that consisted of (I think) Richard Pryor, Robin Williams, and possibly Billy Crystal just walking out on stage where a meal was set and having dinner together? The banter would be legendary.
There was a 1981 pretty-famous movie “My Dinner with Andre” (which today’s post title references), which mostly consists of two old friends getting together over dinner. I think it was a case not of entire improv, but scripted on the basis of earlier improv exercises or rehearsals (or, further back, recollections of an actual occasion like that). Directed by Louis Malle, starring Andre Gregory and Wally Shawn. At the time, there was a loud audience contingent who disliked the movie for its lack of action onscreen. (Plenty happened /in the narrative conversation/.)
Mitch4 (2): Anachronism runs rampant in Hagar, of course.
padraig (3): I’ll bet that was funny.
Then there’s Luis Buñuel’s film “The Discreet Charm of the Bourgoisie” where a group of people have great difficulty getting dinner. Among other mishaps they are invited to a house where they sit down in the dining room, find that the food on the dining table is inedible prop food, and when the curtains part find that they are onstage in a theater and everyone is upset at them for not knowing their lines.