Maeglin and Idril, and marriage

[Artanis]

I wonder if the published Silmarillion is wrong when stating that Maeglin and Idril could not marry, because they were too closely related? While browsing through Laws and Customs of the Eldar, I came by this:

None of the Eldar married those in direct line of descent, nor children of the same parents, nor the sister or brother of either of their parents; nor did they wed 'half-sisters' or 'half-brothers'. Since as has been shown only in the rarest events did the Eldar have second spouses, half-sister or half-brother had for them a special meaning: they used these terms when both of the parents of one child were related to both of the parents of another, as when two brothers married two sisters of another family, or a sister and a brother of one house married a brother and sister of another: things which often occurred.

But Maeglin and Idril do not fall into any of these categories.

L and C goes on saying:

Otherwise 'first cousins', as we should say, might marry, but seldom did so, or desired to do so, unless one of the parents of each were far-sundered in kin.

This is exactly what Maeglin and Idril are: First cousins, and Idril's mother was Elenwë, and there should be no relation between her and Maeglin's father Eöl. So it seems not correct that they could not marry.