Is this a true story? If so, do you have more details or source to read about?
https://www.atlasobscura.com/articles/knitting-spies-wwi-wwi...
It also mentions the subject of OP's article too. I have seen an old piece of knitting from WW2 used to convey messages in a military museum in Portsmouth, UK.
And this link includes details on how to do it:
https://www.popsci.com/story/diy/secret-code-messages-knitti...