Detection is much easier than defense. It is unlikely that any probe could have operated covertly. If testing the ability to reach through the Expanse to Earth was the sole objective, the probe shouldn't have been armed at all, or even manned and should have self-destructed out in the Oort cloud. Any Xindi attack would still need intelligence on the Earth defenses. Read literally, the posts above call for covert testing of Earth defenses! That is impossible. A major attack evolking all Earth's defenses was needed. It is true that the attack gave away information. But what the Xindi gave away was not essential to the Earth defense (that came from Future Guy) while what the Xindi gained was essential to their preemptive (as they believed) attack. A tradeoff in intelligence was inevitable and the Xindi would have won the exchange without Future Guy.