This is exactly the same design they used in the A12X and A12Z ( see: https://en.wikipedia.org/wiki/Apple_A12X )
They clearly do have room for a larger IHS that covers the RAM too, but either this makes manufacturing cheaper since they didn't have to change anything from the previous rev or there's actually a real benefit to not dumping excess core heat directly into the RAM (seems more likely). Sinking heat is great, but only if the temperature differential points away from what you're trying to cool.
(Which is to say, I bet the return of eGPUs on mac will only happen if apple releases a discrete Metal-based GPU)
It makes sense, so far as it means they can further integrate hardware solutions they make themselves and not deal with a higher level driver interfaces. Otherwise they'll be limited to incorporating features present in all supported vendors.
That doesn’t make sense as Metal APIs are implemented on top of the graphics drivers. There’s Metal drivers for AS, Intel iGPU, and AMD. The last two are not built for arm64. On AMD’s side it would require work from AMD as most of the driver is shared code from Windows. I wouldn’t be surprised if that changes in the future.
I can't imagine there's a hard technical reason why they couldn't support eGPUs. I've used both AMD and nvidia Linux drivers on a ppc64le workstation -- I'd be surprised if porting to darwin/arm64 was that much work. Their endianness matches -- though the difference in page sizes might be a problem. On Linux, some drivers just assume 4kb page sizes, and updating them is really difficult[1]. But AMD and Intel GPU drivers are fine with non-4kb at least, so...