From what I understood, the problem was two-fold
It was cold and the band hadn't had sifficient time to rehearse. I don't understand how they jumped from the stated problems expressed by the band director to it being Beyonce, who could probably have sung the anthem in her sleep, even with the cold.
While the modern pop idiom isn't my preferred listening material, Beyonce Knowles is a very talented singer with a remarkable range, and the chops to do just about anything.
The other element is that while she is a melismatic singer (the overly ornamented style currently in vogue) she has exceptional taste and precision in her use of them, and consequently doesn't decend into the chasm of Maria Carey and so many others who cover up defects in voice production and tone with a lot of frippery.