People been looking at Jupiter for a 100 years before the official red spot discovery. Why they didn't make much note of it could be poor record keeping. But it's a storm and one that's currently dying down. It's probably not that much older than it's noted initial observation.
Also op is a faggot once again. It's a storm. Sure I guess there's no way to prove a moon crashing into a planet wouldn't cause a storm. But that's like saying there's no way to prove aliens didn't cause the red spot. There's also no way to prove an angel didn't do it.
It's just a fucking bigass storm. Bigass storm form on a bigass gas planet with no surface friction to stop it being big. Similar big storm on Saturn and other gas planets. Also we observe storms form on Earth and never has one been cause by a damn asteroid cause there's really no logic to a large impact cause a giant long lasting hurricane they happen on their own due to weather just fine.