Power dissipation in MCU
The maximum power dissipation is a rating of how much power the package can physically dissipate. How the MCU ends up drawing that much power is another matter; for instance, it could be simply a lot of GPIO pins each driving substantial output current.

If you want to compute actual power consumption, you should look at the relevant figures in table 10 for power consumption (for instance, IDD(REG)(3V3)), the graphs in figures 11 through 19, and the peripheral power consumption figures in 10.2. Sum up the relevant figures and multiply by the operating voltage to get a figure in watts.