Delidding is a multi step process. Apart from the Tim removal and repaste with liquid metal, there's also cleaning the black adhesive strip from the ihs/pcb. That adhesive also maintains a slight rise in the level of the IHS, so it's removal drops the IHS closer to the die by a couple thousandths of an inch.
The Tim on top of the die isn't all that thick, it's just generally irregular in its thickness which leads to larger differences in temp ranges between the cores.
Delidding drops temps for several reasons, but mostly because of more thermal transfer ability with the liquid metal over Tim, and because of the lowered distance between the die and IHS.
But as advised above, delidding is a last ditch effort, every other avenue of reducing temps should be tried first.