The other reason I was initially confused, but reading this thread helped, is that by presence of bones in the cave they don't just mean that there aren't any bones lying around, brought in by cavemen or bears dying in the cave. They imply that the cave is basically not accessible normally and what would be found in it (bones, fossils etc), if there was anything, can only come from the time when those mountains were formed. I think... Maybe that was obvious for some people.

The original sentence is consistent with the assertion that the mountains had formed before the first boney ancestors of dinosaurs evolved. This is also consistent with the presented timeline.

The correction would mean that the mountains may be older or younger than dinosaurs, because they are only older than bones, and the article is deafeningly silent on the issue of whether dinosaurs had bones.

It's a failure of literacy on the poster's part, hence the downvotes.