Well, Trudeau is the Prime Minister of Canada and is technically the "boss" (for lack of a better word) of those ministers. As the Leader, he has an obligation to set the agenda for the mandate being delivered and hold his ministers to account as Canadians will in turn hold the Leader to account. If that makes him a "dictator" then I suggest you have a very different idea of a dictator than most people. The Liberal Party had made a number of commitments to Canadians during the Federal election and those commitments were in those mandate letters, such as the one to the Minister of Fisheries and Oceans and the Canadian Coast Guard. Trudeau, to his credit, did something that his predecessors never did before - make these mandate letters public. Doesn't seem to be a move that a dictator would necessarily do.
I guess Trudeau could have come up with a different letter for his ministers and called it: "Try to get to it if you have a chance" letter. Like, "Please think about restoring annual federal funding for freshwater research, and make new investments in Canada’s Experimental Lakes Area when you get a chance and if you don't no big deal."....or..."Try to act on recommendations of the Cohen Commission on restoring sockeye salmon stocks in the Fraser River, but if it seems like I am getting too pushy on this because of the lack of traction on these recommendations then I will back off and let you do what you want because that's what Prime Minister should do." Would that have been better? Just leave it open to his minister's own interpretation and self-discretion?