I think that assumes more than can be assumed. If Miller's attorney is correct and Miller was already en route when Miles sent the text, then Miller didn't "get in his car... to bring a gun to Miles." Like I said the in the post you quoted, Miles sent the text at 1:38. The victims flagged down a cop by BDS at 1:45. So we're talking about a roughly five minute span between text and shooting. If Miller was not already en route when the text was sent, then he had to have been very close by, had to have seen the text almost immediately after it hit his phone, and had to have jumped into action very quickly.
That's why I don't think that the text is terribly helpful in determining MIller's culpability. What I think matters is whether or not he heard the discussion between Miles and Davis, and whether or not he drove the from the scene. The text can only go so far in establishing Miller's knowledge. His actions after he arrived at the scene would go much further.