I think they mean there have been 30 reported cases of an astronaut getting a kidney stone less than 2 years after their latest space flight.
Given that astronauts are exceptionally physically fit individuals to start with, at least at the time of their space missions, this is probably quite significant evidence that space flight causes kidney stones.
Given that astronauts are exceptionally physically fit individuals to start with, at least at the time of their space missions, this is probably quite significant evidence that space flight causes kidney stones.