Obviously the luggage was stolen from United, so is United really lying in this case?

If you asked me where my car is, and I said it is at home where I left it, but unbeknownst to me, it was stolen an hour ago while I was away, am I lying about the location of my car?

I don't know anything other than what I did with the car to be true, and it's probable that United doesn't know that someone stole the luggage.

Then you take the information provided by the person claiming that their bag isn't actually there based off perfectly valid tracking method, and start investigating instead of "You're wrong, it's still in our warehouse".

There is no way that whoever this person got in contact with has the power to act on a criminal act that many layers removed from anything support agents can have input into.

Instead of badgering the powerless CS agent I'd go to the police. Cops do not usually respond to a simple "it's in there because my AirTags say so".... but the poster has images of tons of stolen luggage in the dumpster outside. That's easy probable cause and the kind of stuff cops just love to go full cowboy mode over.

Did you see that in the last hour, someone claiming to be the courier responsible for delivering it texted her and delivered the bag?

Very weird that the "thief" would then text her and arrange to deliver the bag.

It was a third-party delivery driver.

Perhaps there was a guitar in the bag?

For the unknowing, this is probably a reference to https://en.m.wikipedia.org/wiki/United_Breaks_Guitars