The real reason the "Xindi" attacked Florida was because of 9/11. Of course, the end, where the attack was all about the Sphere Builders' conquest of Lebensraum, had nothing to do with any notion that 9/11 happened just because Muslims hate our freedoms. Dropping 9/11 propaganda instead of self-flattering fantasy a la BSG (BSG finale: It's us
that God loves, he smote your sorry asses to death!) is one of the reasons people decided they were dissatisfied with the Xindi arc.
Since they were dissatisfied they tried to rationalize it with a logical analysis. One favorite talking point is the claim that it made no sense for the Xindi to "warn" Earth with an inadequate attack by a single probe. Unfortunately, this is an error. There were at least two aspects in the test that could not be replicated at a testing ground. First, there was making the journey from a secure launch site through the Expanse, through the barrier, then through an unknown (to the Xindi) space to a distant planet. Second, and more important, the probe tested Earth's defenses, as well as the weapon.
Sorry, but the insistence that the Xindi attack makes no sense is an error.