This is one of those things that we call: "Situational Writing". The same type of writing that Saul the hulks rage completely stopped by giving him a Hostess apple pie, or showing him a box of puppies. The same way in DC, we're Superman can get punched by someone as physically weak as The Joker, and in the next they show Superman flying into some debris. It's simply a narrative point to show their interaction together and give the reader concern enough to keep reading. There's no way that Groot and any capacity is beating Hulk.