I can't comment on whether it is prophetic or not, but I am pretty sure it is not a recent hoax. Here is a page from the Old Manchu text Jiu Manzhou Dang, detailing a letter sent by the Manchu chieftain Nurhaci to the Khalkha of the Five Encampments in 1620. I translate the highlighted section below.
The highlighted section says (in Old Manchu):
te geli liobe uwen-i gisunde latunahabi. tere liobe uwen serengge dūleke julgei [n]iyalma kai. ganio joriha gese. liobe uwen-i gisun ehe sain ojibe emgeli jorime waciha kai.Which I translate as:
Now again [events] adhere to the words of Liu Bowen. That Liu Bowen was a person of the ancient past. It is like he indicated supernatural things. Regardless of whether his words were of good or evil, what he indicated came to pass.So, at the very least, we can say that in 1620 there was some text attributed to Liu Bowen that was considered to be strange and prophetic. It is possible that the Roasted Cake Song that survives to the present day is a hoax, but unless there is something in the content of the surviving text that is clearly anachronistic, we can probably assume that it dates at least to 1620.