Please refrain from willingly picking the naive interpretation when you've understood my point perfectly fine, it's against the rules of this website.
...sigh:
Secret codes as in "watermark-level omission of characters" are a myth. Lingo and jargon do however exist, and convey meaning in a particularly subtle way. They are shared and taught by culture, not by a secret handbook passed down from generation to generation. See also dogwhistling.
The goal is to protect the issuer, not to selflessly inform the recipient.
"I cannot recommend X too highly. X always served as an example to their colleagues. The quality of X's code was unequalled in our department, and X's work always merited special attention." (etc)
It can be interpreted both ways: "I cannot recommend X too highly (because they suck)" vs "I cannot recommend X too highly (because whatever praise I give will be inadequate)"