Note that this usage of pirate goes back several centuries, e.g. see these entries from the OED: 1668 J. Hancock Brooks' String of Pearls (Notice at end),

Note that this usage of pirate goes back several centuries, e.g. see these entries from the OED: 1668 J. Hancock Brooks' String of Pearls (Notice at end), Some dishonest Booksellers, called Land-Pirats, who make it their practise to steal Impressions of other mens Copies. 1703 D. Defoe True-born Englishman in True Collect. I. Explan. Pref. sig. B3v, Its being Printed again and again, by Pyrates. The term is metaphorical in origin, and in that regard I don't think it's particularly outrageous. If you say that he's "a lion of a man" you're not arguing that the person in question has got large teeth, a mane, or hunts antelope.