no dude, he's talking about HD CRT. I know, it's not clear, I made the same mistake earlier.
The thing is 240 is half 480, yet the black line are really thin on an SD display. You could barely see them on a cheap CRT and only notice them on dark colours. Now, 480 isn't half 720, 360 is. 480 is 2/3 of 720. So you would have even thinner black lines on a 720p screen displaying 480p content. And the fact is you can't see them, except maybe looking very closely at the screen or with a magnifying glass. The other thing to take into account is that the width of the line depends of the brightness of the pixel. It's also why, even though you can't see black lines at 480p on a 720p display, you can still display 720 lines, as the brighter pixels would sort of shine over its dimmer upper and lower neighbours. You don't see the separation because there are no separate cells being switched on and off like on an LCD, the electron beam itself makes the separation.