> Do you have any serious evidence that shows that it doesn't have external validity?
I can't prove a negative, all that a study can do is say "no results observed", but there's plenty of that. From the researcher cited in the article: "Many very specific predictions about the MBTI have not been confirmed or have been proved wrong." "Finally, there is no evidence that the MBTI measures anything of value."
This aligns with all of the other at-all-convincing research I've seen on MBTI: it assigned binary distributions to normally distributed data, and has very low predictive power. The majority of people will be near the middle of those distributions, where MBTI is at its least useful.
Further, I think your example is unfair: MBTI is a one-way function. I can meet people and anticipate their scores to with 1-2 errors (a huge margin, but we talk like it's 'close'!) I can't go from an MBTI score to useful predictions about the person though, because it's so massively information-destroying. Even on the things it purports to measure, the normal-to-binary problem means that it has awful predictive power. MBTI defines a somewhat-consistent function from people to letters, but since it's an arbitrary clustering system it doesn't actually allow new predictions or efficiently compress information about people.
So yes, I could probably tell some people apart by MBTI, but you could also assemble 10 different ESFJ's who are more dissimilar than the ESFJ and the INTP. It's like a compression algorithm that can't be reversed, and about as useful.