He didn't say it proved causation, but it increases the probability of causation. Let's say you have several possible hypotheses:
Watching TV causes worse grades.
Watching TV has no effect on grades.
Something else causes watching TV AND worse grades.
Worse grades cause watching TV.
Watching TV causes better grades.
Etc, for all the other possible correlations between these three variables.
Assume all these are equally likely. That makes it about 1/7 chance that Watching TV causes worse grades (and there is equal chance of the exact opposite.) Now the study that watching tv is correlated with worse grades comes out. You can eliminate all but the first few hypotheses that predicted the correlation. Now the hypothesis "watching TV causes worse grades" has a probability of 1/3. Almost twice as likely. And the hypothesis that watching TV has any positive effect on grades has been completely eliminated.
This is why I'm bothered when people say "correlation doesn't imply causation!!!". No it doesn't, but it significantly raises the probability. If the other hypotheses aren't that likely to begin with (i.e. "cancer causes cellphones") then it should really bring that hypothesis to your attention.
Watching TV causes worse grades.
Watching TV has no effect on grades.
Something else causes watching TV AND worse grades.
Worse grades cause watching TV.
Watching TV causes better grades.
Etc, for all the other possible correlations between these three variables.
Assume all these are equally likely. That makes it about 1/7 chance that Watching TV causes worse grades (and there is equal chance of the exact opposite.) Now the study that watching tv is correlated with worse grades comes out. You can eliminate all but the first few hypotheses that predicted the correlation. Now the hypothesis "watching TV causes worse grades" has a probability of 1/3. Almost twice as likely. And the hypothesis that watching TV has any positive effect on grades has been completely eliminated.
This is why I'm bothered when people say "correlation doesn't imply causation!!!". No it doesn't, but it significantly raises the probability. If the other hypotheses aren't that likely to begin with (i.e. "cancer causes cellphones") then it should really bring that hypothesis to your attention.