Why is it so hard for you to grasp that an analysis of how the paint was applied gives no indication of when it was painted nor by whom. It is a non-sequitur study. Useless! In what world do you think a conclusion can be drawn from:
If "the brush stroke went from here to there", then ...
A. it must be a modern hand that wrote it
B. it must be an ancient hand that wrote it
Neither A nor B are practical solutions.
There is no decisive conclusion that can drawn from the observation. Capiche? Snape's observations are nothing more than illusions meant to appear noteworthy. Creighton tries to add weight to them by claiming Snape is an 'expert'. It is misappropriation of academic input - pure and simple.