It is probably referencing metals such as trace elements that may react in water and form insoluble salts that accumulate in the substrate/filter until some sort of microbial biochemical reaction makes it viable again, if ever. But this will probably happen regardless of the water source you use, since you are probably dosing micros which will end up at least partially in the filter or the substrate. From what it is said, this behavior is common with iron, since it is sometimes dosed in much larger concentrations than actually consumed, as a way to compensate for this known behavior of forming insoluble precipitates.
Tap water may introduce other metals which are not dosed via micro mixes, such as heavy metals. If they accumulate or not, that's beyond my understanding, but plants probably absorb some of them, and precipitates may be removed if they get trapped in the filter or if they are siphoned out when cleaning the substrate...
