Author points out themselves, in the second paragraph, that its not a strange find. The strangeness of the find is his personal experience. Not that its a strange find geologically.
I agree it's not a strange find. Because fossils. But then what was the big deal about finding it?
Remember, the same author says "I found a seashell in the middle of the desert!" "shouldn't be here" and "coastline 500 miles"
And then the author takes a massive leap from “I found a fully solid rock that eerily resembles a seashell” to doing an analysis that treats it as though it actually is a fossil.
And that analysis finds out that the shell the assumed fossil most resembles is completely out of period.