Well buddy, those bones have been cleaned. Thoroughly. There are teeth marks on all of them in various arrays where one can imagine some sort of carnivore was gnawing every scrap of meat off of them. Some carnivores will leave the scrap behind and it'll be cleaned off by fish who swim down to the lakebed; not this one. Most fish do not have terribly sharp little teeth, and don't meticulously chew the bones until there's nothing left.

With that said, they're also old. Mizuki is enough of a little cannibal to know when bones are super old, or at least, not exactly fresh; these ones are lingering on fossil age, and maybe they would be fossilized if they were actually in the sediment. Most of them are human bones. There's a layer of fish bones wedged in between/on top of the massive piles of human bits, though, like the carnivore in this lake hasn't had a human to munch on in some time and has taken to eating the local wildlife instead.