It's actually not that non-standard. It's how the majority of people who are into MBTI / jungian type view things (where I've linked MBTI to Kahneman's Fast/Slow thinking is non-standard, but a cognitive functions + functional stack view is not).
It's also what the more advanced "Step III" version of the official MBTI test is based on. But that that assessment isn't just a simple questionaire: it can be only be conducted by a trained assessor and the cost of training course runs to thousands of pounds, so most people don't have experience of it, and it ends up being the simpler naive model that most people learn about.
If someone did the test and got INTP, then I think it is fair for them to describe themselves as INTP. You might say that the test was wrong, but as I said that isn't how most see MBTI.