The reason why the rice grain method won't work on HDT based heatsink is because HDT tends to have some "longkang" on each side of the heatpipes, thus, it reduces the thermal transfer power.

If you rice grain method, the TIM layer will be so thin, it won't go into those "longkang" which will result to a weaker thermal transfer.
On HDT, you need to spread the TIM all over the processor's IHS, with a lil bit more TIM on the IHS, some of the TIM will be pushed into those "longkang's".

Despite the high performance of HDT technology based heatsink, i found two weakness in HDT:
1. You can't use HDT based heatsink on naked core CPU & GPU.
2. You have to use more TIM compared to non HDT heatsink.

PS:
Sorry, i don't remember what's "longkang" in english. Sorry!