I've dug a bit on the internet. There's a couple other books with this phrase, though it really sounds like a hearsay or a funny anecdote rather than an actual quote.
The book "Charlatan: America's Most Dangerous Huckster, the Man Who Pursued Him, and the Age of Flimflam" mentions "the Burbank of Humanity" three times:
In the first one, the quote isn't attributed:
"the Burbank of Humanity, the Chinese called him"
In the second one, it refers to an unspecified Chinese Press (italics also in the original text):
it was here the Chinese Press hailed their new visitor as "the Burbank of Humanity"
The third one is just echoing that nickname, but in a different context.
Another book, "The Roguish World of Doctor Brinkley" also mentions the "Burbank of humanity" but without reference to Chinese sources. In the following excerpt it seems it's talking about an american journalist:
The early 1920’s marked the beginning in the United States of the new craft of public relations counsel. Brinkley with his Ballous, Flowers and others, was quick to avail himself of the services of various gifted members of the new profession; and who else, but a press agent, would have had the thought, or the nerve, to call Doctor the “Burbank of humanity”?
Baidu is also of no help. First, the name of Luther Burbank seems to be rendered inconsistently in Chinese. I can find at least three versions:
路德·伯班克. Then, we have the problem of whether "Burbank of Humanity" is a direct translation of a Chinese phrase, or the liberal interpretation of some other periphrasis. I have a hard time believing it could be the former, since this kind of noun juxtaposition (e.g. the "poor man's lobster") is not typical of Chinese.
Anyway, even assuming the quote "Burbank of Humanity" is a direct translation, what would it look like in Chinese? Maybe “人类之伯班克” or “人们之伯班克”? Neither these two, nor several other variants, yield meaningful results on Google or Baidu.
Moreover, I doubt that the Chinese press (whatever it may be) would ideate such a monicker at the time Brinkley was said to be in Asia (mid 20's), as for it to appear in press, it would require Chinese readers to understand it in the first place. So this raises the question of whether Luther Burbank, an american botanist, was so renowned in China at that time to warrant calling someone else (a huckster), the "Burbank of Humanity".
In conclusion, I'm led to believe that the quote is anecdotal at best, if not an editorial embellishment altogether.