You may be right, but we have no idea what the scores would have been had Reading Rainbow not been on (i.e., maybe it held off a decline), so this isn't really meaningful one way or the other.
It's true that we don't know the counterfactual: it's possible literacy would have plummeted precipitously starting in 1984 if Reading Rainbow hadn't been a bulwark. But I don't find that the most likely explanation, personally.