Heat is mostly driven by leakage current and gate capacitance.

The big issue today is leakage currents. They typical account for around 30%-50% of total chip thermal budget, and they get increasingly difficult to control with smaller devices and lower voltages. They're also get worse with increased temperature(!).

The stacked devices here aren't the worst for leakage currents, but they're not fantastic either. Look at the 2nd graph in section 5: You'll see that the current never drops to zero over the range of gate-source voltages (for V_DS=0.7V). The minimum point is the best-case leakage current, and you can see it's well above zero! (The units on the vertical axis of the graph are unknown btw: The label reads as "current drain-source, arbitrary units")