Yup, sounds like Archer swallowed Future Guy's lies hook, line and sinker... While the Xindi had no idea what the hell he was talking about.
The device was probing the path to Earth.
Sounds like a good idea. But this should and could have been done covertly, without firing a superweapon at Earth!
The device was probing possible resistance.
The above goes doubly for this. If there is resistance, surely it shouldn't be allowed to witness the Xindi death ray in action! Probing should be conducted so that only the Xindi benefit from it, not the enemy.
What we see of the first weapon is devastating power much in excess of what Kirk's old starship had - and Kirk considered himself capable of sterilizing a planet without having to call in reinforcements. Sure, it's less power than what was demonstrated by the second Xindi death ray, but these were two different tactical approaches, and both had good odds of working. The first would have removed all life from Earth in a matter of hours; the second would have done it in a matter of seconds. Both struck against an Earth undefended by armadas of starships, so we never learned what their comparative strengths or weaknesses vs. defenses would have been...