Morten St George Theory

[Morten St. George]

So it is likely an imagined scene in its entirety

From wiki

The military actions surrounding the Albigensian Crusade helped prepare the expansion of France southward. Philip did not participate directly in these actions, but he allowed his vassals and knights to help carry it out.

Also french wiki says chronicle of st denis done around 1250, completed 1285, it got added to after that as well with things that happened after 1223. It was a compendium of history from fall of troy to life of phillip ii.

Therefore its initial incarnation would not have included info from 1244.

If that illustration was made between 1250 and 1285, it most certainly could have been made by someone who was an eyewitness to the fall of Montségur in 1244, and I think it was. Who else would depict a sling catapult on top of the mountain?