It was mentioned in an earlier thread that attackers are declared before Secret can use its interrupt per playtesters. That said, I tend to agree with Toqtamish's RAW assessment. However, I wouldn't say that it's a solid case as you must have at least 1 attacker for an objective to be engaged (you can't declare an engagement with zero attackers - pg 18). That gives credit to the argument that the objective is not engaged until attackers are declared, which is what playtesters were (reportedly) told.
And that makes no sense with the RAW as it clearly says you pick an objective to engage. So reading that it means the objective is now engaged. I don't give too much credence to the playtesters, no offense to any of them, but who knows what changed after the playtesting period ended. Just have to keep holding our breath for the now late FAQ.