No, I don't think that's true. Certainly there's no skill requirement that appears ( like Tenebrium 4 as on pure T weapons)
Dirigible is right. If you look at weapon damage on the character sheet, it will still say it receives bonus from 2h, 1h, bow, etc., but if you look at the actual number on the bonus it will follow the Tenebrium skill. Any weapon with tenebrium damage on it (whether it's part of the original weapon or it was upgraded by crafting) gets bonus from tenebrium ability, not the other weapon abilities.
