I'm not a pro with this, so some of the science and explanation might be outside my ability to make perfectly clear. Im also typing from my phone, so i tend to shorten things up.
My apologies on advance...
From what I have always heard/read, the white spots that you see are not actually the parasite, they are the skins reaction. Those typically drop off after a few days and this is where people believe that the treatment cured their fish. It's still in the fish, just may not be visible and could very well be causing damage, usually to gils. I've also heard that it is in the system and that basically if one fish has visible signs, any other fish are at least hosting the parasite. Again, it is only visible during one phase of its life cycle, so what you are not seeing does not mean its not there and that it can't come back. It often takes some kind of stressor to bring it back out to where you see those dreaded white spots.
If you "cure" this problem by properly treating it outside the DT and leave the DT fallow for six to eight weeks, you should be totally Ich-free. If you QT any future additions, it should never show up again.