For centuries virtually all the foreigners that Chinese rulers saw came from the less developed societies along their land borders. This circumstance conditioned the Chinese view of the outside world. The Chinese saw their domain as the self-sufficient center of the universe and derived from this image the traditional (and still used) Chinese name for their country--Zhongguo () , literally, Middle Kingdom or Central Nation.
Your sources are weak, and merely rephrase the common cliche. Wikipedia as a source? Give me a break.. And besides, Wikipedia even wrote that Zhongguo traditionally refers to the Central States in the Yellow River valley. Do you know where the Central States in the Yellow River valley lie? The Central Plains! Jhongyuan!
The Chinese never traditionally called their country by Zhongguo as your UMD source states. That's absolute BS. The Chinese traditionally called their country by the name of the ruling dynasty. Thus, under the Qing dynasty, the country was called Qingguo. Under the Han dynasty, it was Hanguo. Your sources are the ones full of crap.