I think they are wrong. It's admissible hearsay. But, it's useless to the state because the credibility is bottom of the barrel.
There is also a question of timing of admissibility. More can come in at the immunity hearing, than at trial. The state will be allowed to present the affidavit as part of the immunity hearing, and the Judge may consider it. Same way the judge considers evidence that is ruled inadmissible to a jury.