Yeah, that is a fair point to make and that is how I figure it goes. But I think it is noteworthy still because while Methuselah was all darkness, killing him together with darkness is still pretty damn impressive. Because it was noted that it would not just end Methuselah, it would end the darkness itself. Which is a bit different from how Reinhard did it, seeing as the latter simply blew away all the mystery within his body. It didn't end the phenomenon itself, but it severed his connection to darkness and returned it to be a simply natural one rather than something divine/supernatural.
There is also another problem: Age. Methuselah is insanely old. And the longer one has existed, the longer one can endure against the MEoDP. Honestly, I am not even sure if she would see lines of death on him. Yes, I say she because Tohno Shiki from Tsukihime has nowhere near as good eyes as Shiki from Kara No Kyoukai does.
Lemme see if I can grab it:
"No matter how many times the material forms are obliterated, the reoccurring phenomenon will ensure his reappearance as a materialized rumor. Even the Mystic Eyes of Death Perception can only kill a single manifestation, even after hitting a point of death. They cannot kill a pure phenomenon, but only single manifestations of them. Conceptual Weapons by themselves are unable to truly affect it in any manner, as Riesbyfe's Apocrypha is a simple substance that cannot interfere with it and even Sion's Barrel Replica can only destroy one part at a time."
Ripped straight from the Type-Moon wiki so not really sure if it is 100% correct but that seems to be one point against her doing that.
But I digress. This is not ODM VS MEODP. This is Reinhard VS Arcueid. And Nilfheimsr Fenriswolf VS her "stats are always higher than the opponent" would make it an endless speed increase. Hell, can she actually power up to such a degree even if the planet is backing her up?