Grey markets exist for MFi chips - especially those Chinese units. Don't see the Kensington, Logitech, and other name-brand cables, do you? Just the Chinese knock-off units. Go check the Apple spec - it's not allowed. Doesn't mean you can't buy some grey market chips and build the cables, but Apple says you can't. My guess is you can't look it up because you don't have access to the MFi Developer network, and thus have to just go by your fanboidom for Apple, rather than the real facts about the Apple spec.
After you pointed that out, I did a quick check for Belkin and Logitech cables. Long story short, you appear to be correct. Those aftermarket cables on Amazon likely are made with bootleg MFi chips.
I am an embedded designer by trade, and technically a registered iOS Dev; but I haven't done any iOS stuff for quite a while, and am not really familiar with the spec you cited, since I was not developing a piece of hardware at the time.
So, bottom line: We were both partially right, and partially wrong: You CAN directly connect a USB-C equipped Mac to a Lightning equipped iOS device without going through an adapter-fest; but to do so "legally" requires the purchase of an Apple-only cable. Although I'm not sure a consumer breaks any laws by innocently purchasing and/or using a bootleg pseudo-MFi cable...