As it has been explained in many questions and answers - look on the right edge of this site the related questions - the development of the well-tempered piano was a product of the development of the polyphony and the purpose of composers to modulate through all keys and accompany  chorals and songs in all keys with claves instruments (cembalos, harpsichords, organs, later the forte-piano and the Hammerklavier. This revolution started already in the 17th century when Werckmeister developed his ideas of the well-tempered tuning.

*Werckmeister is best known today as a theorist, in particular through his writings Musicae mathematicae hodegus curiosus... (1687) and Musikalische Temperatur (1691), in which he described a system of what we would now refer to as well temperament (named after Bach's opus, "The Well-Tempered Clavier") now known as Werckmeister temperament.*

These quotations show that the WTC by Bach has been composed before the first equal tempered **piano**was built but his music has been played by clave instruments, harpsichords and organs.