Thermal pad thickness
Yes, that is correct.

It is unlikely that the surface of a chip or of a heatsink would be so rough that much needs to be taken up by a pad.

Where you might need to use a thicker pad is if you are using one heatsink for several components and some of those components have a different height, and the thermal dissipation does not warrant the cost of machining the base of the heatsink to varying heights. For example, on a PC GPU card.