It's a Titan pokemon. Which means it's catchable with the Titan's ability after you beat it. IDK why they specifically wanted the Titan version to have STAB-Rock and none of the other wild Bombirdier do, but they clearly thought it mattered. They didn't do that with any of the other Titans either(even the Loyal 3).
My no-evidence theory is that originally every Titan was going to have a species-unique hidden ability, but they realized that some(Tatsugiri+Commander, Klawf+Anger Shell) were too vital to their design to lock behind HA, so they had to make those regular abilities, and then they just forgot to make that change with Bombirdier.