Because I just dont see how ARM plays into it. Yes we all know it is rumours. But Mark Gurman has a very reliable record on any Apple Software related news ( His hardware prediction is awful, KGI's Ming is much better in this regard ). So I guess this could be real.
It is not in anywhere similar. It absolutely make sense for Apple to made their own SoC. And for GPU i only question whether it was feasible in terms of GPU patents. (Turns out it was ) Apple Ship 250M iPhone and close to 300M iOS devices, that is more then the total PC market with Intel and AMD combined. And doing it using only 2 Chips per year. The AX variants as pipe cleaner for New Node on iPad Pro, and new A series for iPhone. Compared to a few dozens of variation on PC market from Intel.
Apple ships roughly 25M unit of Mac each year, 5M of those are Mac Mini, iMac, Mac Pro. You could have only a few variation of A10X for the 20M Macbooks. But you need a lot of different CPU for the 5M Desktop Mac market with 45W to 240W CPU. Designing a 240W TDP CPU is completely different to a 5W or 15W SoC. You will use a new 7nm Node for HP, new verification tools, new QA. Two / Quad Memory Channel. ECC memory support and testing. The 20M of Macbook CPU could share many of the uses with iPhone and iPad, but not the same for Mac CPU. That is why it doesn't make any sense financially speaking.
It will be messy, with no immediate upside, and expensive. That is why PeterScott kept saying it is not worth it. And why I suggest a Dual ISA or using Qualcomm Centriq for higher end Mac, or Apple using their own CPU in Datacenter. Because so far none of these make any sense at all.