However, if I were a cynic, I would think that they were just saying it wasn't detectable from the exterior because they missed it during normal inspection (most likely due to poorly trained technicians).
It a good thing you aren’t a cynic, otherwise I would raise a very large BS Flag. Southwest has THE FINEST maintenance technicians in the industry. Each and everyone of them has skin in this game. They all travel on company jets. Their family members travel on company jets. They all, myself included, will go out of their way to fly Southwest vs a brand X airlines because of Southwest’s sterling safety record. A very large part of that is because of our maintainers. In my 20 years of experience, there’s never been an instance of a mechanic short cutting anything. On the contrary, they won’t hesitate to take a jet out of service if it isn’t up to par. Southwest doesn’t do any “poorly trained” anything, at any level. So, choose your words carefully.