Interested to know the several types of edu links as well.
From what I've research though, it will kind of depend if the subdomain of the edu link has been spammed to death by other SEO sellers.
Even if its spammed, most of the time. The admin of the site will delete the post in a couple of months. There's a reason why most sellers don't guarantee more than 3 months because they don't last.
If a seller is selling $15 per link or would accept your article. They are junk, as most people can afford $15 therefore the edu profile has been spammed to death.
On the other hand, if someone is selling you $100+ per post. They are either scammers charging overprice junk or legit.
You kind of have to do your own research about the seller or if the profile where the sample of the edu post is has other junk post.
If they don't take care of their profile, they're 100% junk and are scheduled to be deleted soon.
P.S. - I just check your link, and I would say with confidence the seller is a scammer. The profile has a lot of junk post, those links won't last.