Every signal sensing device works off of DC offset. Many discrete amplifiers don't have enough DC offset to trigger these circuits until they are up pretty loud. The signal sensing is intended to work with deck power BTL outputs and low-performance BTL amplifiers (such as HiFi) and the EP amp is a discrete design.
Basically, deck-power ICs and low-performance amplifiers use bridged channels, which have a ton of DC content.
Their manual does specify this, so it's not like they lied about it always working or anything.