Here's why:
* Hyperbole is a figure of speech that uses exaggeration for emphasis or effect.
* The phrase claims that there was *no one* greater than the person being described, which is an extreme statement. It is highly unlikely that there was truly *no one* greater than the person in question.
The purpose of this hyperbole is to emphasize the exceptional qualities of the person being described. It makes a strong statement about their greatness and elevates them above all others.
Without knowing the specific context of the phrase within "The Tale," it's impossible to determine exactly who or what is being described. However, regardless of the subject, the use of "That greater there was none under sun" clearly indicates a hyperbolic exaggeration.