Although it seems against the spirit of GPL, the GPL doesn't explicitly protect against code hosted as a service, so the wix website isn't in violation. That's why AGPL exists.

IANAL, but their private fork (I couldn't find a public one on Github) is also technically permissible, as it's not a redistribution of the code.

Edit: if what's in question are strictly web assets (images, js, etc), then I'm pretty sure, but not certain, this is a violation

I don't even understand what the point in doing this would be, given they're a publicly traded company. Seems like a very careless decision. If they liked the app, they should've just copied the concept and feel of the app but rewritten the actual code. That way the GPL license would not apply.

How much would that cost? A few hundred thousand?