Isn't there a difference between donated old and new retail clothing? Whereas canned food is more or less the same donated or purchased new at retail?
Furthermore, if you have some money to spend on the effort, buying new cloths is a really poor choice among the options for spending it, for all the reasons they mentioned: organizations on the ground know what items and quantities are needed, often buy in bulk with discounts and, if possible, purchase through businesses local to the disaster, which supports economic recovery.
i.e. donating new cloths is not any better than used cloths and neither are all that helpful; donate cash instead.