Not a chance a Core2duo has a low idle consumption. Computers weren't built for it back then. Just looking at TDP won't work either, as systems that old have discrete north or even south bridges and IGPs. I'd be surprised if idle power is below 80W.
You may be correct about that HP system .. it's hard to find data.
However if you look at this page..
https://support.apple.com/en-us/HT201897... you find the following idle power consumption figures:
13W idle: Mac mini (Early 2009) 2.0 GHz Core 2 Duo, 2GB, 320GB 5400 RPM, GeForce 9400M
14W idle: Mac mini (Late 2009) 2.26GHz or 2.53GHz Core 2 Duo, 4GB, 320GB 5400RPM, GeForce 9400M
10W idle: Mac mini (Mid 2010) 2.4GHz or 2.66GHz Core 2 Duo, 8GB, 320GB 5400 RPM, GeForce 320M
Those are all various flavours of Core 2 Duo.
There was a big power drop once they went to Core i5:
6W idle: Mac mini (Late 2014) 2.8GHz dual-core Intel Core i5, 8GB, 1TB Fusion Drive, Intel Iris Graphics
"Fusion drive" is spinning rust with an SSD cache with dynamic movement of files between them.