Yes, that is what he's suggesting. That EN had asked him to come beforehand and he declined, but he relented the second time after the shooting had taken place.
I don't know how you'd expect any of us to have access to Andrews' texts but as far as why EN would think that, Cameron Frye can explain: "He'll keep calling me, he'll keep calling me until I come over. He'll make me feel guilty. This is uh... This is ridiculous, ok I'll go, I'll go, I'll go, I'll go, I'll go. What - I'LL GO." Also legally speaking it's not required for Andrews to prove he wasn't there, but instead it is up to the DA to prove he was beyond reasonable doubt, so even if the jurors think Andrews is a lying ex-con it doesn't mean the jury will think the DA has sustained its burden of proof.