I think your info is half-a-year out of date, Charity Diary. All weapons benefit from their associated weapon skills. Tenebrium just lets you use those which have Tenebrium in them. Tenebrium weapons also do mostly normal damage with Tenebrium added on as extra.
Ah okay, I must have been thinking of the Wands that only do Tenebrium.
And sorry, it's difficult to get reliable info when the majority of online info about this game is from June 2014. Hardly anyone talks about it anymore. But it totally makes sense that the Tenebrium skill simply allows you to wield certain weapons, while the weapon skills themselves still increase your effectiveness with those particular weapons.
