That's flatly false. Surface area matters. The only other thing they can control is the materials which affects the heat transfer coefficient.
Think about the logical conclusion of what you just said. Can you transfer just as much heat through 1 square millimeter as 100 square millimeters?
Well yes you can - all else being equal, the temperature difference between the two surfaces would need to be 100 times greater to transfer the same heat.
Q = h A ΔT
Q = the rate of heat transfer
h = convection heat transfer coefficient
A = the exposed surface area, and
ΔT = the difference in temperature
You're misapplying your logic here. The surface area of the IHS is of ... well, at least tertiary importance, to its thickness and the size of the heat generating area of the die (assuming good-to-perfect core-to-IHS transfer etc). Heat is transferred most efficiently at high thermal deltas, meaning that as heat dissipates out through the IHS, it does a gradually worse job of moving further into the heatsink, as temperatures drop rapidly as you move out from the spot on the IHS directly above the core. The area directly above the core is always the most crucially important for cooling the core, as that is where the vast majority of thermal transfer into the cooler will take place. The rest of the IHS obviously helps, but increasing its size would see very low returns as the distance from those parts of the IHS to the core would increase. Remember: assuming a similarly sized heat source and the same thermal transfer between the two, your 1 square millimeter IHS will be
much hotter than your 100 square millimeter one. And, of course, that example is ridiculous - we wouldn't be talking something like a 100x size increase, but maybe going from ~40x40mm (1600 mm²) to 45x45 (2025mm²) or maybe 50x50 (2500mm²). Those are 27% and 56% increases,
all of which would be relatively far from the core, as the cores already have plenty of coverage from the current IHS design. So, while a larger IHS would likely have
some beneficial effects, overall it would be very small as long as the heat source remained the same. You would need the IHS to be much, much larger - and also thicker, to aid in the IHS heating up more evenly - for this to have a meaningful effect. But even then it would come into conflict with other factors that would most likely diminish this effect significantly, such as thicker IHSes generally doing a worse job of transferring heat to the cooler - you want the thermal energy to have as short a path as possible to the cooler, after all.
The reason why Zen3 (and to some extent Zen2, and likely Zen4) runs hot is not because its IHS is too small, but because its core is very small, and its thermal density is thus very high. This means the IHS struggles more to effectively transfer said heat to the heatsink, but a larger IHS wouldn't alleviate that in any meaningful way. A thinner IHS could (though too thin and it becomes less efficient at horizontal thermal transfer, counteracting this effect somewhat), as could improved die-to-IHS bonds - or direct-die cooling.
Heck, this is the reason why cooling a 300W GPU is relatively simple - and can be done with even a 120mm AIO and a reasonably powerful fan - while cooling a 300W CPU is near impossible: GPUs spread their heat evenly across a large die and don't have IHSes, while CPUs have very small cores and have IHSes to protect them.
The only reasonable way of improving this is to improve the IHS materials. I've been speculating for years already if we'll ever see vapor chamber IHSes, and IMO that's not unlikely the way thermal density is going. Obviously it won't be trivial to make such a thing in a way that would survive the mounting pressure of a cooler on top of it, but it would be doable. And, of course, it would significantly increase BOM costs for CPUs.
(All of this is also of course dependent on the cold plate design of the cooler - an IHS is essentially necessary on any direct heatpipe contact HSF, as otherwise you risk the heat generating cores contacting just one heatpipe, which would lead to terrible cooling overall. On the other hand, a HSF with a soldered cold plate, or a water cooler with a finned cold plate, would cool far better without an IHS than with it.)
Then I really don't understand why users praise chiplets so much, considering that the end product costs the same as traditional, monolithic CPUs. It's only AMD's gain, not ours.
It's only due to chiplets that we actually get 6 or 8 cores at reasonable prices though - through going this route, AMD has forced Intel to cut into its production margins significantly. Before chiplets, we had 4c4t and 4c8t CPUs in the ~$150-400 range for nearly a decade, while now we get 6C CPUs for less than $200. That's a major value improvement no matter how you look at it.
Of course, per-die costs are a pretty small part of a CPU's price overall - don't they generally hover in the upper double digits range at most?
This is why I hated the R5 3600 that I gave away after a week or so. Either the IHS design hasn't improved despite the switch to LGA, or chiplets will always run hot. 
