Yardley's "Device" to Convey a Secret Message under an Ostensible Plaintext?

I'm interested in encryption that allows more than one reading (see posts about Venetian, J.F.W. Herschel).
Yardley tells such a feat in The Chinese Black Chamber posthumously published, though no details are given (p.152-153).
He was teaching his Chinese students how to evade the censor in sending out a secret message in an apparently innocent message encoded with the standard codebook for Chinese characters. Whether the message passed the censor could be known by addressing it to a fictitious recipient in Hong Kong, because they got inquiries as to the unknown addressee.
When the censor (a general) came to confront Yardley directly, the ostensible message he questioned was "Please tell my cousin to send me airmail two hundred quinine tablets." while the "secret message, encrypted by a device I showed them" was "New airfield completed six miles up Little River."
His interpreter demonstrated to the general how to derive the true meaning by using the public codebook and succeeded in satisfying the censor.
To the astonished Yardley, the interpreter explained that he revealed a fake secret message, "Your first born is a male.," adding that "I can make it say that as easily as anything else." Actually, he didn't understand the method himself but he knew the general (censor) wouldn't understand it, either, and he was too proud to request further explanation by saying he couldn't understand!