I totally get your perspective on this, and I think your analysis is closer to Garak’s character. He was always very sly and subtle.
But the joke was funny so I’m happy to let it slide.
My interpretation wasn’t that he was going to kill a random person though, it would be more like this:
Words said: I am going to kill someone and it will be totally random
Interpretation: Garak is not going to kill anyone because he’s just a tailor and that was a joke
Reality: Garak maybe is going to kill someone but it won’t be random at all because this is an assassination
DS9 did leave the door open a lot of the time about whether Garak was still involved with the order or not so he does have that genuine air of mysterious uncertainty about him.