It's unknown whether Bach wrote the WTK for equal temperament (the modern standard) or for a well temperament (something that tempers all keys enough to be usable but does not make all keys sound identical).
50 years ago, unless you were studying at the highest levels of theoretical/historical music research, you'd likely have been taught wrong.
The “Bach standardized the world on 12TET” trope is old and enjoyable enough to make a good story that unspecialized music teachers have parroted along for generations.
We've got better access to information now. I've corrected music teachers on this specific topic in the past. Some gratefully accept. Some pull out the “but the teacher here is me card”, so I'm sure a few more generations are going to be needed.
