"It's a relatively new thing, only available with phones preloaded with iOS 12 or newer."
So as i understand, iPhones including Xs and after should all have a "last charge" date right out of the box.
Perhaps the manufacturing date of the device.
That would be different than on a brand new phone. When the factory makes the phone, before they ship them out they charge the battery so you won't have to charge it just to be able to set it up initially. And that's why It shows the last charge.
If you disagree, what do you reckon is the answer?