Does seeing the results affect your belief?

If you take as a given that the HN population is not representative of the population at large, then it is clear that Myers-Briggs measures something, as it correctly identified that.

If you'd asked me for a hypothesis before the survey, I would have predicted a significant overrepresentation of NT-types. That would seem to give the test some predictive value, unless you attribute the full significance of the findings to selection bias in who chose to answer (in which case Myers-Briggs would be an outstanding predictor of who will answer surveys).