I think that the medieval art article is making a different point. The art there had a style that was dictated by its purpose and the beliefs of the artists.
For example, most of the examples given in that article are illustrations from manuscripts. This was something (as far as I know) that was new in the western world. The idea that books should be illustrated. And being before the printing press was introduced, each illustration (of which there were often many per page) was hand made. This added a substantial amount of time to an already labor-intensive process. And each image was not intended to be a standalone work of art.
Also, some of the other examples are of iconography. That style remains, largely unchanged to this day. If you do an image search for "religious iconography", you will see plenty of examples of sacred art that are not visually realistic but are meant to be metaphorically or spiritually realistic.
Sure, but for me the standard isn't whether it's visually realistic. Plenty of good stuff isn't particularly realistic. Traditional Chinese landscapes aren't realistic, but a lot of them are great. David Hockney has a lot of good work that isn't realistic and uses primitive-looking technique. The standard is not realism or which style was used. The standard, for me, is whether the artist was any good at art. Hockney is. (Usually.)
I'm not particularly basing my opinion on the examples in this article. It's easy to see that a lot of surviving European medieval art sucks. Maybe "surviving" is the problem. Maybe the good stuff got all smokey from being displayed and only the leftovers and student paintings, in storage, have survived.
On illustrations, everybody can see the difference between Durer and most medieval stuff. It's not simply style or taste.
So, just to make it clear… you define good art by “whether the artist is any good at art”.
Illuminating…
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For anyone who’s interested in a slightly more nuanced take on how people in the Middle Ages perceived of “art” — and how different that notion was to how we perceive it today — Forgery, Replica, Fiction by Christopher Wood [1] is a really interesting read.
Here’s the last sentence of the Goodreads summary, which describes the major transition in thinking:
“… Ultimately, as forged replicas lost their value as historical evidence, they found a new identity as the intentionally fictional image-making we have come to understand as art.”
[1]: https://www.goodreads.com/book/show/3921524-forgery-replica-...
It's all good and spiritual, but it seems that they lost some artistic tools like point-projection perspective during non-that-well-documented ages.