I think it's a jokey way of saying that. nuggets are not really pure meat. I'm sure the 90%/ 10 percent figure is exaggerated here, but I'm also fairly sure that in a chicken nugget there are are quite a few other ingredients: coating, cereals, flavourings, additives, preservatives etc. So that what you are actually eating is quite a long way from pure meat.
I think you can describe this kind of exaggeration as 'hyperbole'.