Yes, I've read it in this Digital Foundry article, some time ago.
http://www.eurogamer.net/articles/digitalfoundry-2014-has-microsoft-fixed-the-xbox-one-scaler
The question is how they did it, but it all points out to the fact that the hardware upscaler is programmable? So you create an algorithm, send it to the upscaler and it interprets/apply it.
Maybe. It's just an assumption.
But from the start the sharpening filter was a very rough algorithm without even basic image reconstruction or edge anti-aliasing. So the hardware must be very, very limited.
Either 1 - they reprogrammed the old scaler to do some very basic bilinear interpolation upscaling similar to the PS4 scaler (but why didn't they program then to do some basic edge reconstruction and such?)
Or 2 - The sharpening upscaling was done by an additionnal cheap hardware (not programmable) and they simply bypassed it by using the AMD GPU own scaler.
I would tend to support the second hypothesis.