There is no benefit, and significant risk, in lying about that (it's an easily disproven claim if they're lying).

I don't trust them because I like them. I trust them because being truthful is in their best interest (and I trust they will always act in their own interest).

It is not in their best interest, since it's obvious that for at least a decade, blatantly lying and even being caught lying has no impact on their so-called credibility. Lying about this does improve morale among troops and citizens, and gives them some form of early justification. So no. They do benefit from lying about this.

They why haven't they done that in any previous round of attacks? The risk of loosing credibility and giving Iran the opportunity for a propaganda win significantly outweighs any benefit to lying about this.