I think you are laboring under a slight misapprehension about personality research. Myers-Briggs isn't solid science. The eneagram isn't solid science.

Your understanding is consistent with mine. Myers-Briggs is really frustrating, because some of its ideas are anecdotally compelling (Introversion vs. Extraversion, Thinking vs. Feeling), while others are esoteric (Judging vs. Perceiving and Sensing vs. Intuition). At least on the types, INTP probably refers to a real phenotype (which is common on LW), but I don't know if any of the other type combinations are real.

Interestingly, the MBTI seems to almost reduce down to the Big Five according to this study.

Big five personality traits are kind of like that. From what I've read, they're better understood as mostly-orthogonal surface regularities with causal explanations from many different levels and sources rather than as fundamental causally coherent essences. Lots of people seem to expect human traits to coherently cause human behaviors, so it is worth emphasizing how liable such thinking is to produce error.

The way I've heard it explained goes something like this: "you don't like art because you are high in Openness. You are high in Openness because you like art."

Of course, since the Openness scale has reliability, you can make predictions about how someone would respond to one question from the scale if you know what they would respond to another item. Whether that's because of one underlying trait, or because of a bunch of converging traits, is an empirical question.