this is the best explanation to this question so far

Don't cancel. I've heard when you cancel law schools just automatically think you had a horrific score.

If you try to look for the meaning of the sentence, you would realize that the "only" part is actually modifying the "that were eventually written down" part. You could rewrite the sentence to make it like this: The oral myths that have survived are only the ones that were eventually written down.

so therefore the translation would be

survived -> written

it wouldn't be

written -> survived like you said because just because a myth was written down doesn't guarantee that it has survived to this day.