That article doesn't really explain it.
In a word, by multiplexing. The human voice carries frequencies of perhaps 200Hz to 10kHz (but the voice phone system has always truncated this to something much narrower than this - which is why the voice sounds different over the phone). But the copper or fibre cables carrying the signals are capable of transmitting frequencies far higher than this. So frequencies way up above what can be transmitted or heard by humans are used to carry the data that brings broadband down the final distance into your home.