Thermal conductivity of Nickel is 90.9 W/(m·K).
Thermal conductivity of Copper is 401 W/(m·K).
The electroplated nickel on the copper block is so thin that it can virtually be ignored when considering thermal conductivity.
It would be stupid to make a block out of solid nickel. Its thermal performance would be terrible.
Silver is only 7% better than copper but is much more expensive. Pure silver also oxidizes quicker than copper. About 10 years ago, when silver was much cheaper, there was a heatsink
, the Noise Control Silverado, that used a 50 gram slug of pure silver.