echo
略

OPERANDS
The following operands are supported:
string
A string to be written to standard output. If any operand is -n,
it will be treated as a string, not an option.
The following character sequences will be recognised within any of
the arguments:

\a Write an alert character.
略

-nはオプションじゃなく文字列として扱え、
以下の(\)シーケンスを認識しろ、

と書かれてるね。