That's her defense attorney' take on the situation, sure.
Having worked in a store similar to wal-mart, and dealt with situations in which the police were involved, I can say with little doubt that something else happened here. What exactly, I don't know. But the police would never have become involved from a 'lame cashier' alone, or the cashier and their supervisor alone, or anything like that. For her to be arrested, the cashier, their supervisor, the front-end manager, the store director, and the police themselves would have to all conclude that she needed to be arrested.
If this had happened in my store, it could only have happened in very particular circumstances. We have a few cashiers that are in one way or another special needs. Some of these cases are obvious. It's not super common, but not completely rare either, for customers to attempt to bamboozle these cashiers into giving them shit for free, or at unfairly reduced prices. Some of the time, it even works.
I know of several semi-professional scammers, and plenty more opportunistic, entitled douches, who would absolutely try to pass off a bill like that to such a cashier. No one has tried a million dollar bill at my store, but fraudulent rebate checks, phony smartphone apps, homemade coupons or lies about Canadian currency have all happened, sometimes successfully. Often, the liars don't exactly premeditate, they just try to browbeat the challenged cashier into giving them their way.
They'll say things like "Hey! Hey! Yes, You! Don't you dare pick up that phone, you have a customer right here talking to you. That's incredibly disrespectful! You're a disgrace!" to prevent them from seeking help. They'll get angry, make up emergencies and terrible consequences should they be forced to wait. They'll claim refusing them is illegal. They'll threaten to call the police. They'll try all manner of trick and bullying to get their free shit at the expense of a mentally challenged cashier who's sincerely proudest achievement is their one-year perfect attendance certificate.
Anyway, the only way I could see anyone at our store calling the police over this would be if it was a situation like that. Of course, even though these people could get away if they'd just walk out--the police's investigation begins and ends with running the licence plate if someone happens to have it--they invariable stay to bluster, defend themselves, threaten to sue, use connections, or otherwise promise to make the store "sorry" for the ~40 minutes before the police actually show up.
Like I said, that's just one example, but I honestly can't see anyone being arrested over the events as described by her defense. A lot of people would have to decide it was worth a lot of trouble to see her punished before a case like this would progress to the level it did.
Of course, stores have plenty of cameras especially at the registers, so there should be plenty of evidence of what happened. I'm just saying, I'd bet a fake million dollar bill it wasn't as innocent as the defense is making it sound after the fact.