No, they don't.

The code is available so anyone can modify as they like. XT is a great example of how anyone can just fork, release a new code and (if the majority of miners like it) be the code that rules the network.

Only because some one wrote the code that the majority uses now, doesn't give any absolute power over what might happen next. Of course their saying has some influence, but the choice is still for the miners (individually) to make.