I'm out of my depth in discussions about art history, but comparing the 18th century illustrations to the 15th century painting in the same article, I find it hard to infer a notion of progress as you do. Having said that, one could well imagine that the 18th century illustrator was deliberately attempting to evoke a medieval style.

Here are some links picked up from quick Google searches that might provide stylistic comparisons:

18th century - http://www.theparisreview.org/blog/2015/06/09/charlie-charli... 14th century - http://io9.com/bizarre-and-vulgar-illustrations-from-illumin...