Yeah I think it fits m with the style of the show that it’s a joke that never gets explained because there isn’t actually an explanation. Just a way to highlight a cultural difference and Welsh eccentricity.So basically it's a bit like what happened on the trip that no-one speaks of. We'll never really know why we're laughing along.
And the fishing trip will never be revealed - because it would always be a let down. They are exploiting the power of the “not said” - always more effective. Like how lots of horror films are usually scariest before you actually see the ghost, when you just see their impact -a noise or shadow or whatever.