This has also been my impression.So in short, you get a lot of people who ask "INTJ or INTP?" and are told that "INTJs are doers and INTPs are whiners". Or something to that effect, more or less politely phrased. To hear INTJs describe themselves, you'd think they're mostly a bunch of ExTx with some IxTj thrown in the mix. Which they probably are. At any rate, the descriptions of Ni as a sudden "a-ha" feeling backed up with certainty (!), as contrasted to INTPs' constant doubt, definitely discouraged me from considering it prior to socionics. All in all, regardless of the infamous Lytov's table, I do suspect many if not majority of online-typed INTJs are socially introverted SLEs or LIEs.
"Definiteness and self-confidence" describes ExTj and ESTp, not INTj. This does not mean that there aren't areas in which INTjs are self-confident or that they don't ever appear "definite", but as a general description it is extremely misleading.