How to measure a cup of "solid" ingredient?
It's asking for exactly what it says: 8 cups of broccoli. Measuring by volume is certainly best-defined for liquids, but it works fine for solids too. It works best if you have a large measuring bowl - or just a bowl that you happen to know is around 8 cups (two quarts, about two liters). Or you can just guess by eye, knowing how much volume that is. A recipe with broccoli isn't going to fail if you're off by a bit.

(If you really prefer weight, this nutrition data says a cup chopped is 91 grams, so 8 cups might be a bit over 700 grams.)