They're still fresh and bone-looking. The guardians have been dead long enough for their remains in necrum to be fossilized,
according to alf . If they had been fossilized those bones would look more calcified and grey. Also, they would not be sitting out in the open like that otherwise they would have eroded away over the years.
Even if you don't agree with me on that, you're still yet to say anything in favour of them being Guardians beyond the fact that they're both
really big. We know that there is a significant amount of megafauna on Oddworld. There's no reason for those bones (or the ones in the levels in MO) to be anything we have seen before.