That's because part of it is still on and has a battery/supercapacitor backing it. The above post is complaining about this.
They’re actually complaining about the fact that it still keeps some wireless services turned on when the device is off, for example to find your phone or to use nfc for public transportation. I don’t think anyone complains that it keeps track of the time.
That would be because there's a battery on your motherboard keeping the RTC alive.