But there is no german verb called "shlingen".

Does the person who wrote this, speak german?

I assume "schlingen" was meant, but this sounds more like "etwas herunterschlingen" aka eating quickly; that was mentioned via "scarf down, as in food", but to me it sounds more like a description of the object that is held here, like a sling.

But why would that be associated with Vienna, anyway? Nothing like that exists in Vienna really. Also I am not sure about that explanation; to sling makes more sense with hurling something in this context. And the thing looks like a huge slinger. Nobody is opening the mouth to eat everything coming out of that thing right?

Well, they didn't mean "schlingen" obviously, so I guess they made up a new word :)

Vienna is probably because of Wiener Würstchen, i.e. the hot dog sausages.