Quoting elsewhere in this very thread:
it's mentioned in the Camel, in the "Ideographic Unary Operators" section, page 92:
Unary ! performs logical negation, that is, "not". See not for a lower precedence version of logical negation. The value of a negated operand is true (1) if the operand is false (numeric 0, string "0", the null string, or undefined) and false ("") if the operand is true.
So I guess that boat has sailed not just practically (which it did a long, long time ago) but also as far as being officially documented. Larry did have some editorial control over "the Camel", no?