Yes, though only to an extent that it can be. Touchwiz can't be turned off because it's more than just the home screen launcher. It's everything underneath that including all of Samsungs air view, smart gestures, stock apps, Samsung services including print services, push services etc...
All of those will run regardless of what launcher you choose to run. And in some cases depending on home screen launcher you decide to run this means you can actually have more memory overheads than you had with the stock home screen management.
Basically folks need to stop thinking of Nova and similar as a 'full' launcher, and more as a home screen management application. Because it doesn't replace the entire android launcher, it merely replaces the home screen management portion of that.
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