I fly a lot for work and I also do a fair bit of failure point and risk analysis as part of my job, so this is interesting to me in a couple of ways. Airports and airlines honestly do a decent job of checking that the people on the plane are the ones who are supposed to be there. A failure like this is reasonably unusual.
she got through physical security (baggage and carry-on checks)
to accomplish that, all she had to do was dodge the ID and boarding pass check.
That seems pretty feasible. If she was dressed vaguely like an employee it might have helped, but that's just speculation. We've all seen the gorilla walk through the ball game - after we were told to look for him - so it's not strictly necessary.
I have a harder time understanding how she could have boarded through the passenger line where they scan the passes.
I also have a slightly harder time understanding how she could have found a plane with open seats. I can view a seat map 12 hours ahead of boarding and see a plane with 10 open seats. When it comes time to board they're completely full. But - part of this is because the airline shuttles regional pilots to their main hub via any available seat and they do it at the last minute. And here's my further speculation: a flight from Nashville to LA is a long haul so this shuttling probably wouldn't come into play. If she checked seat availability in advance, it probably would have been accurate and she could probably help herself to a seat that appeared open.
The final hurdle seems to be the one that caught her. The article doesn't say exactly, but it says that authorities were waiting on the ground. Stewards have a flight manifest that lists every passenger by name and by seat. On rare occasions I've seen them checking the manifest as passengers board - for example, on overbooked flights where they've sold steward seats for take off and landing to passengers and they expect stewards to squat in the aisle. I've also heard anecdotally that if you're acting like a weirdo they'll look up who you are.
tldr: I could (and do!) give zero fucks about who won Sunday's sports match, but can conceive of why it might be news, of of interest, to some people.