I don't even think it's heatsink compound or thermal glue, i think it's just that "selastic" material , silicon compound that hardens after application and holds stuff in place, used just to prevent vibration of the components inside the case.
Doesn't do anything special for heat transfer and should be obvious by looking at the component legs that they intend to isolate the parts from others and if they block leads with those things they don't really care much about heat transfer.
Note that some of these solutions as they dry out from the heat they can become slightly conductive and cause problems ... so if you see the substance dark yellow / brown that portion could be a bit conductive - if the amp. has issues, may want to separate the part from the heatsink and break any potential paths that could affect circuit (like if substance would cause a short across the diode leads which doesn't seem to be the case here) and test again and see if that solved something