Earlier quoted context omitted.
By that standard, the U.S. liquid measures could also be described as "metric". 2 cups->pint, 2 pints->quart, 2 quarts->pottle (rarely used, more commonly called a "half gallon"), 2 pottles->gallon.
You appear not to understand the actual utility behind the page width/height ratio used in ISO/DIN page sizes. Also, the word "metric" has a very precise meaning: that of being part of the internationally agreed system of decimal measurements. The thing you've mentioned don't fit that definition, and US customary units are simply defined in _terms_ of metric units, but are not metric units themselves.
I do understand it. It's exactly the same reasoning as being able to divide liquid measures in half.