I'm pretty sure that's not true. Disabling WC24 on a Wii only stops your message board and the Shop Channel (the latter of which the WiiU has its own version of...dammit); doesn't interfere with an individual game's online. I know because I went through a panic-boycott of WC24 during one of their Terms of Service revisions (Wii's network being competent enough to enforce stuff, it's all so laughable now...).
Now I haven't actually tested this, but from what I understand the absence of WC24 on the WiiU would prevent you from sharing stages, replays, and screenshots, since all of that stuff works through the System Menu's message board. For the same reason I'd imagine Friend Credits in Metroid Prime 3/Trilogy are shot. But to actually play a match online, and manage your game-specific Friend lists, that all should work fine on the WiiU.
Also on DeviantArt, Rumble, DLive.tv, and the Fediverse (@freespeechextremist.com and @bae.st)