Yes, it’s from a book called “Uncle John’s Bathroom Reader” and that book was resting on a closed toilet seat in a nation that only kind of uses the metric system in certain situations… however, you’d be surprised how much you can learn after a single visit to any local fast-food joint!
And the fact that Inch is not precisly defined. “Three grains of …” or “King’s thumb” is not a precise definition.
Nowadays, the inch is defined in metric units anyway.
Without metric inch is arbitrary unit, can be anything.
Precise definition isn’t really practically necessary. It’s only useful if you need to independently recreate a system in isolation. It’s much easier to have a reference material and just copy it. You can even build a reference length if you have something with known measurements.
One ten millionth of a quarter of the circumference of some random ass planet somewhere isn’t either.