I think the key word above was modern. I felt able to design a simple CPU when I finished my Computer Architecture course in university. I think I forgot most of it by now ;) There are a few basic concepts to wrap your head around but once you have them a simple CPU is doable. Doing this with TTL or other off the shelf components is mostly minimizing/adapting/optimizing to those components (or using a lot of chips ;) ). I have never looked at discrete component CPU designs, I imagine ROM and RAM chips play a dominant part (e.g. you don't just built RAM with 74x TTL flip-flops).
He probably used off-the-shelf RAM chips, after all, RAM is not part of the CPU.
In the early 70s, before the internet, even finding the information needed would be a fair amount of work.
I learned how flip flops worked, adders, and registers in college, and that could be extended to an ALU. But still, that was in college, not high school.
I've read some books on computer history, and they are frustratingly vague about how the machines actually worked. I suspect the authors didn't actually know. Sort of the like the books on the history of Apple that gush over Woz's floppy disk interface, but no details.
