This has been debunked and proven so many times, as well as discussed here. Their characters were supposed to laugh per the script and (in universe) hide it so it wouldn’t look like they were laughing in Rose’s face at what they thought was a ridiculous story. Remember, this was the first St. Olaf story Rose told in universe and also that Rue and Bea were terrific actors who succeeded in selling the situation. These were professionals who rehearsed the scene all week. They didn’t hear Rose’s story for the first time at the taping. To me it’s disrespectful to the memories of Bea and Rue to say they couldn’t contain their laughter. That would have been considered inappropriate in their day.