Whether P = NP is a long-standing problem. Like many long-standing problems, it's deceptively difficult, which results in many people incorrectly claiming they have solved it: https://www.scottaaronson.com/blog/?p=458

A lot of cryptography relies on NP problems being difficult to solve. If someone figures out a way to solve NP problems in polynomial time, they may keep that technique to themselves rather than publish it. The rest of us might only become aware of this after we see real-world crypto being broken.

In this particular case though, it was just another P = NP paper that was rejected, but accidentally got published.