Does this mean that the protist is engaged in more complex activities than the multicellular organism? If not, why should the protistan cell be much more complicated?
Protist cells engage in all the same activities as multicellular organisms do - they have to get energy either by photosynthesis or by consuming organic molecules, they have to respire, they must maintain water balance, they reproduce, and so on. The big difference is that protists have to do all these different things with just one cell, whereas a multicellular life form can engage in cell differentiation, a process which allows specific cells or groups of cells to become specialized for a specific task.
Because of this, when we compare a protist to a cell from a multicellular organism, the protist cell seems to have more complexity. However when you consider all the different types of cells that it takes to make up the multicellular organism, you can see that multicellular organisms actually do more activities, they just spread those activities out across a number of different cell types.