Radio waves down a wire are by definition AC. Otherwise they don't work. The voltage must change, thus the current must change, thus it is AC. Just because it's not 50/60Hz AC like a powerline doesn't change that.
There might be a DC component, but ultimately all the ADSL modem cares about is the AC part.
Couldn't you send an amplitude modulated signal down a DC line by modulating the voltage but keeping that voltage always above zero? If the current never actually reverses, it wouldn't be AC as I understand it. It would be PDC, pulsed direct current.
There might be a DC component, but ultimately all the ADSL modem cares about is the AC part.