He's full of sh!t.
Hook a tv up to the cable line and see if you can get basic channels. If you dont then they have a filter on your line to cut out those channels/frequencies when proves him wrong about "seperating the two channels"
If you do get channels then he is full of it because you signals wont change if you did officially get cable.
The modem works just like a tv tuner or radio tuner. It only looks at certain frequencies and ignores the rest.