From the last time I saw it, I did get the feeling that our heroes did believe in the "test" theory, not that it made much difference in their actions - but that the Xindi never did. They had tried and failed, and wanted to try again. And they now knew Plan A didn't work, so they launched several others, including the "Carpenter Street" Plan B and the megaweapon Plan C.
Transcripts from: "The Expanse"
[Temporal communications room]
(a humanoid silhouette is in a cone of light)
SILIK: He can see you more clearly if you move closer.
ARCHER: Who is he?
SILIK: He wants to talk to you. It would be foolish to ignore him.
ARCHER: What do you want?
FUTURE MAN: Your planet was attacked.
ARCHER: I'm aware of that.
FUTURE MAN: What you're not aware of is why. The probe was sent by the Xindi. They learned that their world would be destroyed by humans in four hundred years.
ARCHER: How would they know what's going to happen in four hundred years?
FUTURE MAN: They were told by people from the future. People who can communicate through time.
ARCHER: Are they the ones the Suliban are working for?
FUTURE MAN: The Suliban work for me.
ARCHER: So you're the one who tried to start a civil war in the Klingon Empire, the one who's manipulated my mission from day one.
FUTURE MAN: The people who have contacted the Xindi belong to another faction.
The probe was only a test. The Xindi are building a far more powerful weapon. When it is completed, they will use it to destroy Earth.
ARCHER: Annihilate us before we can annihilate them. Why are you telling me this? FUTURE MAN: The Xindi were not supposed to learn of their future. If they deploy this weapon, it will contaminate the timeline. You must not let that happen.
ARCHER: Why should I believe you?
FUTURE MAN: You have no choice but to believe me.
Transcripts from: "The Shipment"
GRALIK: How long are you going to keep me here?
HAYES: That's up to Captain Archer.
GRALIK: Well, you can tell him that I'll be in my study.
HAYES: I'm afraid you're going to have to stay right here.
GRALIK: I have work to do.
HAYES: And I'm under orders. Please sit down.
GRALIK: I've already told you everything I know. (door opens) Ah, Captain Archer. I was beginning to think you'd forgotten about me. (Hayes leaves)
ARCHER: You said you weren't involved in building weapons.
GRALIK: That's correct.
ARCHER: Explain this.
GRALIK: I don't know what this is.
ARCHER:
It's a fragment of a probe that murdered seven million people. The dead pilot was Xindi, the alloy is Xindi, and it's imprinted with the quantum signature of the compound you produce here.
We know you're constructing a larger version of the weapon, big enough to destroy a planet. I want to know where it's being built!