That's what warlocks do. That's not what fel magic is. Wowwiki isn't outdated at all. Warlocks drain life from things. That doesn't make spells they cast fel. So you could also say that death knights use fel magic then with Death Siphon and Death Pact. And this is from Wowpedia
"The ultimate manifestation of the demonic arcane on Azeroth is fel energy. This energy, which most commonly manifests itself as ghastly, green-yellow flame, is arcane magic at its most corrupt, for it employs the blood of demons."
And I guarantee you if you simply asked "Sean Copeland, what is fel magic?". Nothing else. Just using those words, he would tell you "It's demonic arcane magic. It's arcane infused with demon blood.".