Why should I mount the HMC462LP5 Testboard on a heatsink ?
(As recommended in the datasheet, page 6, bottom)
5V * 67 mA does not look like excessive heat.
You are correct. Worst case power consumption is around 0.5W. The thermal resistance is 52 degC/W. So when the ground paddle is at 85 degC, the die temp will be at 110 degC. Assuming that the eval board has reasonable thermal resistance, I don't think that additional heat sinking is necessary. I believe that the text used in this part of the datasheet is boiler plate stuff which should be corrected/removed.