from what I understand, yes, it is.

The structure of straighter hair allows moisture to be sealed in longer because the outer cuticles lay flat like this: _ _ _ _

Where as curly hair has a twisted "cork-screw" pattern that bends and lifts the cuticles like so: ////// which allows moisture to seep out at a faster rate (causing dryness sooner and more frequently).

yeah, some people feel they need to beat up their hair with shampoo.
When I shampoo I put tons of conditioner from my ears down before shampooing to protect the length from the chemicals in the shampoo and shampoo my scalp only with a low-soap shampoo and it cleans just as well as the next without sucking the moisture out.

My wash-routine:
-twice a week
-detangle under shower with cheap conditioner (V05) and wide-tooth comb, rinse out.
-Elucence conditioner on length pre-wash as a protectant
-Nature's Gate low-sulphate shampoo on scalp only
-rinse shampoo out through the ends (even with cond. on it's enough to clean my length)
Condition:
-after-wash conditioner Giovanni 50/50 mixed with unrefined coconut oil.
-Deep conditioner: Biolage hydrotherapy mixed w/coconut oil and a heat-cap.
I always wash my length w/shampoo b/f deep treatments so I get the full absorption benefit.

style:
-Giovanni Direct leave-in
-raw shea butter on ends for sealing
-L'oreal lightweight Melting gel

I have wavy hair vs. kinky hair and still, I suffer from dry hair by the end of the second day after washing it. sometimes I have to wet the ends and re-moisture in b/t washes.
I personally believe anyone with curly hair will benefit from using moisture in every step possible.
People with straight hair also have the benefit of their natural oils being able to travel easier down the hair-shaft. But I am happy I don't have straight hair...you just can't do anything with it and when it gets greasy it sticks to your scalp. Not a good look, moisture or not.

I'm with you in the fact that I like my shampoos to rinse away the dirt and just the dirt. I don't want it to affect the moisture balance in my hair one way or the other which is why I use balancing shampoos.
Alot of the "moisture" some people think is in their shampoo is just silicones which sits on your hair. It's used to make your hair feel "nice" temporarily (the illusion of health vs. healthy) but if it builds up it could damage it in the long-run.

p.s. that Dudu Osum shampoo looks so delicious. Unfortunately it has alot of lightening ingredients which is the opposite of what I want. I'll probably try it anyways.
I've always wanted to try black-soap as a shampoo. I've heard some who've had success with it.