yes, it is very true.
however, if you QT in an appropriate QT tank with hyposalinity, they won't get ich again unless they're exposed to it.
I had a hippo that initially had ich, treated it with hypo. then I added 2 more fish (at different times) without QT'ing them and introduced ich both times. hypo'd again both times.
finally, I started hyposalinity QT'ing ALL new fish (which basically means around 2 months in the QT). but now, I've got all my fish and I have no fear of any of them getting ich because they've all had hypo and I don't plan to add anymore fish.