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!
That’s not the point I’m trying to make. You can derive a length from any number of physical constants to make it reproducible, but considering the divisor of exactly ten million suggests the metric committee were far more interested in making something ‘clean’ than something human-friendly. They could have basically done what they do with the modern redefinitions of the metre - the distance travelled by light in 1/299792458 ths of a second. Obviously they chose that number to keep the definition in line with the original, but they could have done that with the original definition to make the metre basically any length they wanted, but they didn’t.