1995 GMC Suburban
In the original design, that may be the case.That's a pretty specific alloy to be using to make a cosmetic cover, wouldn't you think? Considering the environment where this device is designed to work, in the hot valley of a V-8, I'd bet those thermalloy covers over the power transistors are there to shield them from external heat by shunting that heat around the cases of those transistors to the heat sink to be cooled by incoming fuel. The idea would be to limit the heat the transistors must endure to the heat they generate themselves while operating.
I am not sure if they put any thought on the composition of the cap anymore, though.
In my case, I took it off also before I mount the PMD into the heatsink.
I also use the heatsink compound like the one used in Computers like Arctic Silver.