It’s actually a funny oxymoron if you unpack it - agent implies autonomy and intelligence, harnesses are for unintelligent beasts of burden to control them.
So you’re right, it makes absolutely no sense and would be hard to decipher, but does perfectly capture the contradictions inherent in attempts to treat dumb LLM word generators as intelligent independent agents.
I don't think it's an oxymoron at all. An expert climber still uses a harness for safety, even though they won't (typically) try to jump off the wall.