Author claims ENFJ is opposite of ISTP in MB. I thought it was the groupings (first two, second two, and/or/? combination) that formed opposites, not the individual letters.
Taking the individual letters' opposite is pretty common. The interpretation of the resulting opposite differs somewhat by system. (This is getting into what's known as four-letter Jungian type, not specifically MBTI)
There are also temperament groups, which are NF, SP, NT (HackerNews' audience...), and SJ. Generally you don't want streams to cross here with the type of information people of each temperament want to consume and the time perspective they prefer (past, present, future), though there is some overlap.
Some would say it's a good idea to get at type opposites by mirroring the functions, e.g. ENFJ (FeNiSeTi-FiNeSiTe) would be functionally opposite to ESTJ (TeSiNeFi-TiSeNiFe). So these two type groups would tend to want to repress' the other's perceptive and decision-making functionality. The same would not necessarily be as true, or true in the same way, with e.g. ISTP and ENFJ, which some would consider a potentially lovely pairing due to shared function-valuing, meaning that these two opposites will naturally develop toward the other over time, from a developmental perspective.
I think Isabel Briggs-Meyers had her own view of the first four functions' order, which you can read in her book "Gifts Differing". This would also yield a different type of opposite.
