Bach didn't advocate equal temperament, never mentioned it, and probably never saw or played an equally tempered instrument.

"Well tempered" does not mean "equally tempered". This is urban myth.

It's stark staring obvious with The Well Tempered Clavier, since the pieces have very different character depending on the key they're written in, with far-away-from-C keys being used for extreme effects (listen to the E flat minor prelude and fugue from book 1!). If he meant equal temperament they would all sound the same.