My reading is:
1) Someone (person A) said "He's not the messiah, he's..."
2) Another (person B) thought A meant that Terry Jones played the messiah (Brian) and corrected them by writing "...he's the actor that played his mum".
3) I stepped in to say, I don't think A implied Terry did was the messiah in the movie, he just referenced one of Terry's lines from the movie.
Makes sense?
Was there some joke I've missed?
“Analysing comedy is like dissecting a frog. Nobody laughs and the frog dies.”
- Barry Cryer
But I considered it premised on being an (actual, regardless of if being done as a funny quip) correction of the grandparent at the same time.
Let's analyze this further, it's fun!