When he says it's almost impossible, he means that it is.
To remove a vocal track from a mixed-down song, it would require that you remove a certain frequency range at all times. The only problem with that is those frequencies are shared by other instruments. Also, it will end up making the vocals sound like they're singing through paper towel rolls.
If you want to get just the vocal track, then you need to just either re-record everything else or you need to get the original MULTI-TRACK recording. With this, you can just take out whatever you need to take out. That is, unless it was recorded live, then you out of luck.
As he said, impossible. Just about.
With Garageband? Lightyears past impossible. Pro Tools would even give you a LOT of trouble to just "take out the vocals."