Yeah, you got it.
The point (again!?!) is that the area to be cooled is very small (inside the block, about as big as the core), and that may not be enough surface area for the phased substance (R-134A) to remain liquid while sitting on top of the hot area. If it's not entirely liquid, then it's partly vapor, ergo the heat transfer is not as efficient.
Under that logic, one would push to have the coolant cool the block, and not the block's hotspot. So, make the block's base plate thicker, so that this hot spot issue isn't so much of an issue anymore.
Even simpler, such a block only needs to have a baseplate that is solid (maybe even polycarbonate) but with a copper part that touches the core.
It'd be tricky to do with polycarb though, without endangering the core.
Maybe something like this:
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