They are and Genshin literally confirms that + Nibelung isn´t even confirmed to be descender. And if you gonna bring that will argument then I can bring up that each sinner is confirmed to have the will to transcend the entire world.
Wrong, he is stated to be stronger than teyvat, many times stronger than teyvat that is a description vastly above any descender description who are merely stated to rival the world. Not only that he is stated to remove the curse of immortality by his power, the same curse that was established in higher level of existences and he is even stated stated to be a higher being and power.
Proof? Because right now the Abyss is the strongest force in the game, Even the H.P and the shades are rendered powerless in the abyss.
unsealed Traveler would lose to Surtalogi due to him being a lower existence than Surtalogi.
Where? Because Surtalogi is as free as a bee

he is literally out of his free will traveling space