Descriptions only have what text is added in the meme. Therefore you would have yo flag the image since the descriptions are only the words you added via the imgflip meme maker, which is why you would flag the image.
However, I see your point. You can make text that’s unreadable on the image itself by making the text box extremely small. If you add enough text to an extremely small text box, the text becomes invisible. So sometimes worrying messages can be found only in the description.