Sorry, but I don't think this proves much.
Because it is unlikely, that the Customer-Support would know about this.
They are not involved in on-going development of new devices / firmware and usually do not have much more information then is publicly available anyway. And even if they did, they would certainly not be allowed to share this information beforehand.
Sure, it is possible, that the dark mode will be made available for more kindle devices. The upcoming Paperwhite 5 is most likely going to have one, since the PW4 already did. Maybe that's what the support guy was talking about here...