The thing is, that particular caller/witness (the one who advised that KA wanted to kill MW) was anonymous and refused to be identified. They didn't just request to be anonymous. So I would think that would have to be substantiated in order to escalate a charge?
There's another part of the affidavit where another witness, "Jane" (the one who advised that MW had an on again, off again romantic relationship with CS, once again contradicting his self serving statement of his "one week" relationship), that witness requested to be anonymous so she was given a pseudonym in the affidavit. But LE likely has her real information. The caller who anonymously reported KA's comments and rage toward MW, did not disclose their identity to LE. That may have been enough for an arrest warrant, combined with all the other evidence, but I don't know if that claim would stand up in court without more evidence or proof that KA said those things.
I don't think this is the case at all, but for all anyone knows (or what the defense could argue), it could have been a friend of CS or MW, or someone with an agenda, etc etc.