Some people need structure to get things done. Other people find structure limits their ability to do things. The latter usually happens because their imagination exceeds the imagination of those who create the structure.
But why is he so confident that he would be working on something of particular interest to him?
Because they are explicitly inviting him to come hang out and work on anything that is of interest to him?
It's like two CS guys who both have an interest in solving big problems efficiently. They have ideas. They talk to each other about their ideas. They really love working on their ideas. One of them gets a ton of research money so he asks the other to come work with him in the new lab allowing him to work on his ideas as he wants.
Because they've probably discussed the possibilities beforehand...