This statement is true. John Stuart Mill does say that pleasure is really the only thing that people seek as a goal for the actions that they take. Mill says that it is not really possible for people to seek anything other than pleasure. He puts this very clearly when he says(quote can be found in the Stanford link)
to desire anything, except in proportion as the idea of it is pleasure, is a physical and metaphysical impossibility
Mill is not saying that people are and should be hedonistic. He is not defining pleasure simply in terms of physical desires or anything like that. However, he is saying that all people seek pleasure and that therefore we should act in ways that maximize the pleasure/utility that is available to all people.