It is a bad analogy, because the problem with studying marathon runners is that they've already used the fitness technique you want to study. They're runners because they walk and run a a lot. (And not necessarily because they're young! People run marathons in their 80s.)
This study is about people who've never used Luminosity's techniques, but are in unrelated good (brain) health. There are a lot of known ways to mitigate or undo damage to the brain. Luminosity made a much bolder claim, that they could raise IQ in healthy, high-functioning adult brains. That's the Holy Grail of intelligence research, because no one has ever found a way to do it.
It's like claiming a two mile walk each day will make literally anyone a much better runner. If you claim that, it's relevant that your offering is no good to marathoners.
This study is about people who've never used Luminosity's techniques, but are in unrelated good (brain) health. There are a lot of known ways to mitigate or undo damage to the brain. Luminosity made a much bolder claim, that they could raise IQ in healthy, high-functioning adult brains. That's the Holy Grail of intelligence research, because no one has ever found a way to do it.
It's like claiming a two mile walk each day will make literally anyone a much better runner. If you claim that, it's relevant that your offering is no good to marathoners.