Thermal impedance too high for proper use
The efficiency is given as a typical figure, so reduce it a little for a safety margin.

Output power: 20 W
Efficiency: 0.85
Input power = 20 W/0.85 = 23.53 W
Waste power = 23.53 W - 20 W = 3.53 W
Temperature rise = 3.53 W * 17.6 K/W = 62.1 K

Resulting case temperature at 20 °C ambient = 82 °C.

The data sheet says the maximum case temperature is 105 °C so it will work.

However I would not like a component to be that hot (e.g. it might severely reduce the life of other components near it) so I would be looking at adding a more substantial heat sink than the optional one and some forced air flow if it was going to be run at that heat dissipation.