It doesn't say he wrote it hastily. It's not just a note jotted in the margins like the AAm Balrog numbers thing. You're right, there is no measuring stick we can lean elves up against to determine their greatness (besides burrahobbit's graph, which I hope is merely the first of a series). But we can trust Tolkien when he says, unequivocally and without contradiction, that Luthien was the greatest of the Eldar.
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