To classify as kibble I think it would need to offer enough all the nutrition required to survive of it alone. You're not going to live long on just cereal
And not all that bad, actually! I'm dead serious, there were times when I caught myself sneaking out a fistful from my cat's food bag and munching like a rodent. Once you get accustomed to the taste (which, in most cases, tastes like really tough bread flavoured with saltless steak, or saltless boiled carrot, or saltless fish), it makes for a decently filling snack.
Dog kibble has even less flavour than cat kibble, mostly tastes slighly savoury and bland, though. Which I find weird and, like... all dogs I've met enjoyed a plethora of flavours, wouldn't they do the same with their kibble?