The result seems bloody obvious to me as a dancer. Dance is exercise. And this wasn't just dance, they were learning moves and choreography. Like, no duh, teaching people new and complicated things increases neuroplasticity! According to the quote there, the activities were matched in physical intensity and one treatment added a significant mental component versus the control.
Compare dance to rowing, lifting, spinning etc. Those activities are regularly accomplished by a brainless motor. That such activities induce neuroplasticity is cool, but it's no shock that more enriching activities are better for the brain.
I think it's obvious that a younger person's brain would be more improved by this class than your ordinary seniors athletics program. I'd be more inclined to compare with other low-impact competitive sport: badminton, table tennis, etc. Like dance, those require full-body coordination, planning, reflexes, etc.
Like saying the heart worked harder with people on a treadmill at 4 miles per hour than 3 miles per hour.
I think there is also this bogus implication then that dancing would help you write a better novel or painting a picture than just walking on a treadmill.