So I read through the article and here's a explanation.
The estate owners are salty that they don't get any money from the characters they helped make, they're throwing a hissy fit threatening to do this, but it was legally covered in a contract when they worked at Marvel. Most likely they just want money (TL;DR)
The explanation is that they created these characters, but since they were employed by Marvel to do so, the rights (at least partially) belong to Marvel as they were hired and provided the necessary tools to create them, thus they have a legal right to use these characters. The copyright duration lasts for 56 years (Under the Copyright Act of 1976) and "termination could be taken into effect at any time within a five-year window if the works were created before 1978".
Spider-Man, created in 1962 has his copyright termination window from 2018 to 2023, so that's why this has been brought up. Another similar case was brought up but it was settled on a cash agreement before going to court. I should also note comments from them stating that they've only received "Thanks" and a small token gesture of $5000, to which they rejected on the fact it was considered a joke to them. It's unlikely that they'll tear down any Marvel projects and most likely will receive a hefty cash sum to shut them up.