I don't know what's going on... has buying a CPU now become a lottery as to whether it runs crazy hot or not?? Or is it simply early batch issues? Unacceptable whatever is happening.
My understanding is a little different, the der8auer video is a good explanation of it and to be honest that's really the focus of his entire 9900k delidding video.
Basically there's 3 materials and each has a different level of conductivity. The more conductive the quicker heat will get through to whatever cooling you're using. There's the chip itself, there's the TIM and then there's the IHS. The IHS he explains is copper so it's high conductivity so that's not really the issue. There's the TIM which is the solder and it's quite low. And there's the chip itself which is somewhere in the middle of the two.
The new 9900K has a much thicker chip, and the solder is a thick layer. That means he saw a fairly impressive temp drop by getting rid of the thick solder for thin liquid metal. And he saw further gains by lapping down the CPU core. He did 0.2mm and said he could have gone deeper.
I might end up lapping my 9900K down by 0.3 or so mm because this upgrade is intended to be my last for a very long time, so if I can drop temps by 20 degrees then it'll make a lot less fan noise over the years.
tl;dr they'll all run hotter than 8700K would because there's much more material that's less conductive than there is in an 8700K. He's not quite sure but he's wondering if Intel had to make other compromises to get it soldered and as a result it's cancelled the gains.