You're right, it looks like that is a myth (I remember seeing it on this math stackexchange question[1], which is actually a great example of some mathematicians' opinions on the matter).

There's a wide margin between "couldn't pass high school" and "had difficulty with math in high school". And I've known plenty who confessed that were quite explicit that was not what they meant (e.g. by comparing themselves unfavorably with their children).

[1]: http://math.stackexchange.com/questions/551074/are-all-mathe...