I think the grandparent was just quoting what Jones (as the mum) said in the movie tho, not saying he played the Messiah: "He's not the Messiah, he's a very naughty boy".
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.
Yes. The joke was that Person A's comment, replying to an article about Jones, makes it seem as if the 'he' in the line is referring to Jones, not the baby Messiah from the movie. B then subverted the expected conclusion of Jones' line "...he's a very naughty boy" with a fact about Jones himself: "he's the actor that played his mum". Person B was aware of Jones' line and played off it.