They didn’t Miranda him. Maybe they didn’t want to Miranda him.

What?

Miranda warning is for custodial interrogations, they don't have to read it to you if they aren't asking questions or if its non-custodial (you weren't under arrest). And only those statements made under a custodial interrogation would be inadmissible, everything else would still be admissible, including the gun and manifesto that they supposedly found on him.