My tuppence worth: In most tanks whitespot is endemic but the incidence is below 'visible' presence. Even though it can remian dormant for 4-5 weeks in substrate, it must be finding some host in order to keep the reproductive cycle going, just below the radar for us tank keepers.
If this is the case, on any one day there is likely to be a stage of whitespot looking for a host to keep their reproductive cycle going. The resistance of the potential host fish restricts this.
The debate would be about whether a WS theront would be capable of latching onto a fish, burrowing into the skin, growing, and forming a lesion visible to our eye, in a matter of hours.
Plenty of other stuff to throw in; some writers believe theronts/tomites are most active at night which is why fish can seem clean in the morning (theronts have exited fish/ cleaner shrimps have had a better chance to clean fish) but infected later in the day when previously invading theronts have developed lesions.
I think there is a degree of support for your view HazyM cause we probably all know of fish that have 'developed' whitespot within a day of being put in a new tank. The question is how long it was already developing on the host fish.