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Leporello recites a catalog of the Don's "seductions," totaling 2065. We see from the beginning that Don Giovanni is willing to actually forcibly rape women in his "seductions." Even if the vast majority of the 2065 were mostly consensual (albeit with lots of deception and lies), how many outright rapes has he committed? 50? 100? It puts the colloquial use of "Don Juan" as a sexy suave guy in a whole new light.
It didn't help that at the time 'being in a position where nobody could help' was often treated as 'consent' and there was precious little concept of consent given under duress. Just looking at the YMMV page for critical treatment of Anna over the centuries should give you some idea of the sort of attitudes such victims would face. (Not that it's entirely a thing of history, of course...)
The heroes know Leporello as Don Giovanni's devoted manservant, aiding his master in all manner of trickery and deception. They've finally tracked down Don Giovanni, and burst in. No Don, but Leporello is there. They ask him where Don Giovanni is. Leporello replies (approximately) "uh.... the Commendatore's ghost appeared and dragged him straight to hell. Yeah, that's the ticket!" The heroes' response? Believe everything Leporello just told them, say "justice had been done!" and break out the champagne to celebrate. That Leporello's crazy story happens to be true doesn't make this any less implausible.
There are lines in the libretto which suggest, and don't fall far short of outright stating, that Elvira met the Commendatore outside as she was leaving. So Leporello's story isn't that implausible in context, although it is true that after earlier events in Act II the other principals have every reason to distrust anything he says.