It's sound advice.
Most fish are carrying parasites and can be completely asymptomatic in their behavior and appearance. It's the cause of many fish's sudden and "strange" deaths.
The common visible signs of parasites are scratching on sand or rocks (flashing), low body mass, spots on body, and/or stringy feces. There again, a fish may still be infected with no signs at all.
QT, observation, and preemptive treatment for the common parasites is the best way to a healthy fish and tank.