No, that’s the die sawed out of a wafer. Those are wafer-thin, i.e. about 0.8mm. On some newer CPUs the wafer is ground down to improve thermals. In normal C4 packaging like here the active layer is sandwiched between the metal layers (which sit on the interposer PCB) and the bulk silicon (95% of the thickness upwards, to the camera). Silicon is not a great heat conductor.