I have done lots of free-form couples dancing (over 20 years, several times per week, some of it in complex styles like argentine tango) and in my first-hand experience the major difference from other kinds of exercise is that you need to harmonize your movements with your partner and music, and you need to improvise. The complexity of the movement is not it, nor the "exercise".

There is lots of other research suggesting that couples dancing is more beneficial to the mental health and memory than other kinds of exercise.

That said (and having no background in an very little knowledge of MRI research), I am also skeptical like you that there would appear such clear visible signs so soon in the brain images.