It's actually a lot easier nowadays to build you own silicon. Good tools, libraries and cheap FPGA's make the process cheaper. Still expensive making the actual silicon, even on a MPW.
I'm not surprised in this move TBH, getting a custom designed chip exactly targeted at what you want, rather than buying something not quite right from a third party for more money seems like a good move.
I don't think this is overstretching. Its not like Musk is sitting there doing the design himself, he has employed a lot of smart people to do that for him.