@gaigous Technically, "piracy" is defined simply as the act of robbery or criminal violence on the sea, so it could be argued that the mere act of stealing the ship makes them pirates, albeit sky pirates as opposed to traditional seafaring pirates.
That said, I went back and skimmed the previous chapter; the ships don't exactly seem that uncommon, as there's a side note that mentions that air pirates can operate legally so long as they don't attack merchant ships or carriages (so, presumably, this world's version of privateers) and the one they come across is merely the most famous air pirate. I agree with your assessment on the quality of air gems being the main roadblock for air ships being far more common, but with regards to the ship's barrier being incapable of defending against the ship's ammo, I'd stand by the logic that the return shot was far faster and more powerful than the initial shot, which is why it broke through... Of course, this is a comedy series, so we're probably doing something wrong by overthinking it so much.