Hello. I am not an expert. But I went to read the species description from the 1800something paper. There is nothing to suggest it has a penis or some kind of reproductive organ of remarkable length. Instead, it does seem to have very long wings, and since cockroach forewings have a pennate structure, it stands to reason to think that the perpendicular venations in the wing are long, because of a combination of angle and length available. Therefore longipennis.

Now this is just guessing, so of there is someone who knows better I'd love to hear about it.

It's because the scientist who named it had a fucking long dick. Thus the name.