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?