Nobody can confirm whether a device might be counterfeit without a look at the product and its software, possibly with tests, and appearances and serial numbers can all be copied.
A “great deal” from a sketchy vendor isn’t an auspicious sign for an authentic product, for instance.
Battery health doesn’t get interesting until it’s at or below 80%. Battery health of 90% is about half way through the battery, but battery degradation isn’t a linear process. Exposure to excess heat or physical damage can accelerate the decline, while some batteries can be “stuck” at a health percentage for a very long time.