Kinda weird line, to me. Curious if it was for connecting a third party USB drive at all, and if so, why mention the porn.
It seems like companies either have a strict USB policy, or don't. If they allow USB drives, it feels odd to me to fire someone over a drive containing porn. What if it was a genuine mistake of confusing two drives or something? Or a genuine drive that happened to just contain something copied by accident.
But if OP was say, copying, streaming, or uploading the files, then that seems like a more clear violation than what the article states.