It's not about the roots of the English words. The pun is that x and y are relatively uncommon letters in English, that when they appear before the "ing" suffix, it's easy to accidentally parse them as relatively common Chinese words.
The "xinger" is that "hoaxing" has a 腥: "fishy (smell)". ;) [1]
A quick Google of "hoax" suggests it's possibly derived from the 17th century noun "hocus".