If an engineer is outstanding, their actions can speak for themselves, and they won't have to put on airs; if an engineer is mediocre, their only option to hide it will be arrogance.
There's a third type: the engineer who is outstanding but is addicted to being perceived as brilliant, so they layer extra arrogance on top, never conceding points, overstating facts and contributions, etc. to make themselves appear even more brilliant. When I encounter these types, I think to myself "If you just didn't feel compelled to overstate things, you'd be truly awesome."
If an engineer is outstanding, their actions can speak for themselves, and they won't have to put on airs; if an engineer is mediocre, their only option to hide it will be arrogance.