In this case, it was a scandal that happened in the past. Something very strange when the case was raised again after so long. I think this is only one of the "dirty" way to bring down Boris Johnson.
Maybe. But you can't dismiss it just because it happened so long ago. It was a different time, we cared less about it then, we're trying to care more about it now. There are ways to verify these things, such as contemporaneous accounts. And as far as I can see Mr. Johnson doesn't really need dirt to be brought down, he's doing that just fine on his own but I'm not that well-versed in British politics so I apologize if I missed some nuance here.