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'.

The original meaning of nuggets is small pieces of gold that you find in certain areas when you break up rock looking for gold... These are the nuggets that most people know about today 😊 -->