I believe a NAND gate is required as the base on which all other possible circuits can be built, so they need just to add an inverter for potential turing completeness.
It's not, we just use NAND everywhere because they're easier to make with transistors. You can get functional completeness with a NOR instead, or alternatively with some different combinations of other logical operators.
We even implement AND gates with NANDs in electronics (because they're way simpler), but we might not have to limit ourselves to a single base gate with mechanical computers.
> Where do you think the N in NOR and NAND come from?
That makes it sound like you could also do a NOT with XNOR, which is only the case if you can use a constant 0. But that would similarly also be the case for a XOR, but with the requirement of a 1.
