Yep. 5e gnolls take fluff from 4e, in which they were hyenas corrupted by the demonic essence of Yeenoghu, and lose all of the aspect about retaining hyena souls and thus being able to turn their back on their demonic origins. s5e gnolls are, really, little more than zombies; they're essentially a kind of low-level fiend that lives on the material plane, avatars of Yeenoghu's insatiable hunger.
They're literally so evil that telepathically contacting one has a risk of driving you insane and turning you into a rabid, cannibalistic worshipper of Yeenoghu, at least in the fluff. So, in a way, they're more like the movie version of a zombie apocalypse than the gnolls of editions past.
I'm one of that small minority who vehemently detests this change in lore and sticks to the 4e version, if you can't tell.
I don't really have any idea. I was kind of thinking that the gnolls might handwave that in their mythos, or acknowledge the existence of other gods in making the Young World, but not worship them, because those gods have no investment in the gnolls. Does that make sense?
As for the pleasure thing... I don't have a good in-universe explanation for it. In traditional D&D lore, gnolls are basically described as being inherently lazy, for no particular reason. A hedonistic streak was my kneejerk idea for explaining this.
Basically, gnolls as a people lived the hunter-gatherer's lifestyle for some time. That's not an easy life. Pleasure was something you're lucky to get. So, they seized upon the idea of pleasure - or, rather, access to pleasure - as a sign of status. If you can afford to lounge around, then you're important enough that you don't have to struggle for food, sort of thing.
This is something I've struggled with to explain. The core idea probably remains; gnolls just don't like working physically all that much, and if you don't have to work, you need something to fill that time.