Actually this is not true. Equal temperament had been known for about two milennia before Bach’s time but he didn’t use it, and equal temperament is not the meaning of “well tempered” in Das Wohltemperierte Klavier. Bach blended a pair of unequal temperaments for his keyboard tuning. He didn’t like equal temperament, and it was not popular until the early 20th century when it became the normal piano tuning in Europe (records of piano tuners confirm this) starting around 1913. It was at first highly chromatic and later 12 tone composition technique of the Second Viennese School, Shoenberg, Berg, and Webern, that made equal temperament an absolute necessity for piano tuning.

The standard system of guitar fretting does not actually allow accurate equal temperament – that would necessitate tuning each fret at each string, and there are some approaches to this – but it does approximate equal temperament with varying degrees of success depending on set-up.

The common fretting scheme using the “rule of 18” (or 17.8175 as many builders use) has been around since at least the late 16th century. Lutenist and mathematician Vincenzo Galilei, Galileo’s father, used it for placement of his lute’s gut frets and he advocated it in spite of its being wildly unpopular with professional performing lutenists, who preferred a variety of unequal temperaments. Their use of guts frets tied around the neck allowed them to move the frets and re-temper based on the keys in which they were playing. Same thing on the renaissance and baroque guitars but that ended with the advent of metal frets in the 19th century.