Exactly, why don't just wait, this would make no sense from a risk rewards perspective
My phone rarely goes below 10%, and maybe it has seen 5% one or two times. If Facebook waits for that, they will never run the tests. But if they detect my battery is at 10%, then drain it to 3%, launch the controlled failure testing, then drain the phone to complete shutdown, on the next boot they have their very valuable data.
Also there are hundreds, if not thousands, of different phone models. The allegedly used way of negative testing is much better than buying phones, and is virtually free.
Handling low battery states isn’t exactly rocket science anyway. Do less computation. The rest is up to the OS.