keep in mind that Ich isn't always visible, for instance any Ich in the gills. the white spots you see on infested fish are actually the 'exit wounds' of the parasite that was under the skin when feeding and then dropped off to cyst and split.
I forgot the exact stats, but the percentage of fish that carry at least 'some' Ich is a quite high percentage. It is high enough that it is easier to just assume they have Ich rather than not. The high immune fish are just able to battle off the high infestations, but is likely they still have a population on them somewhere, just not enough to irritate them.
You won't like this next comment, but, given you are starting down the QT path now, I would highly recommend you pull that lawnmower and QT him through TTM as well. Run the DT fallow for 10-12 weeks. Since best practice is to QT and observe fish for a total of 8 weeks, you are only adding an extra 2-4 weeks to your timeline; minor in the big picture.
Anything new you add to the tank that is wet (live rock, inverts, coral, etc), best practice is to QT them fallow for 10-12 weeks as well, to ensure that any Tomites (Ich cysts) have hatched and run their course.
this will start you off on a near-perfect course for an Ich-free tank for life. well worth the time investment, IMO.