Nope, I don't think megafauna made that. Sure there were carapaces discovered but I doubt their claws are strong enough to dig through stone that thick.
I completely agree with you. For me, it looks like some of the indigenous inhabitants carved it. By the way, are you sure that it is stone? Even if it is stone, there is a possibility that it is not a hard one. May be a soft type of rock such as Limestone or mudrock. Because even if the natives carved it, we need to remember that they didn't had access to metals.