It is a mistake to think that radiation is evenly distributed. Or that all radiation is equally harmful. Neither are all animals and plants affected in the same way. This is much more complex than some averaged random statistics will have you believe.
Not that I disagree with what you said and I would even add that children, adults, and the elderly all have different cancer risks as well. But, a sievert is normalized based on harm and not the energy equivalent for any one radiation type. So, while the risks are different it is plenty useful for evaluating order of magnitude risks as in is this directly lethal, a significant cancer risk, a minor cancer risk or a negligible risk.
A Facebook phone makes no sense. So it seems probable that this is just one or two new apps for Android. Similar to their remake of the iOS apps a while ago.
