Actually SLDL's are a superior lower back movement, along with GoodMornings and Romanian versions . Attention is to a straight back, without any unneeded rounding of the back. Gluts/hips, hams and even calf's are stabling units within the lift. And as suggested before (by myself..too many times), the weight is held in position by the upper body while the legs do the actual pushing. But like the regular DL, just holding a heavy weight below the waist builds mass in the total back area, including the traps.
A lot can be said about just holding a heavy weight in place as far as strength and development are concern. Even Hise Shrugs affect not only the traps but the whole chain of muscles all the way down to the feet. Can give overall power to the body. Try holding a crucifix on a pair of gymnastic rings for a few seconds and add more seconds each time you do them. The whole upper body, acting as a unit, will become stronger, even the abs. Or try holding a BB(or DB) overhead for any length of time and notice increase strength in the whole chain of musclesdown to the feet. Plus added shoulder girdle flexibility.
My best SLDL is about 130lbs below my best regular DL. Which only suggest how strong that lower back area is when geared up for heavy lifting, for anyone. Good Luck.