"However" isn't a conjunction. It should be "Not many academic philosophers discuss magic. However, five centuries ago..."
Using "however" the way the author did will trip up a reader expecting correct English -- once you have "Not many philosophers discuss magic, however" you expect the sentence to end, but it doesn't and you have to go back and reinterpret.
""Not many academic philosophers discuss magic, however, five centuries ago, prominent Renaissance philosophers, such as Marsilio Ficino (1433–1499), Giovanni Pico della Mirandola (1463–1494), and Giordano Bruno (1548–1600), wrote extensive treatises on the topic""
I think you'll find a lot of people would breeze past this and not debate commas and semicolons.
You are correct, by the letter of the law, according to the semicolon police this should be a semicolon.
But that is like arresting people for jay walking. Yes, it's wrong, but come on.
I agree it isn’t going to trip many up, but also agree that the standards of writing coming out of an education institution should be grammatically correct.
But this is super low bar mistake. I would bet 90%+ of general population would not know this was an error at all.
But, what is prevalent today, is to pile scorn on to educational bodies. So to take a really minor grammar error, in a rather small and niche paper, and extrapolate to a condemnation of the entire institute, is really obviously politically motivated.
Using "however" the way the author did will trip up a reader expecting correct English -- once you have "Not many philosophers discuss magic, however" you expect the sentence to end, but it doesn't and you have to go back and reinterpret.