He wasn't witnessed by the McMichaels. He was witnessed by a neighbor who called them. The certainty with which the McMichaels may have known he was the same guy from the prior security video, the same guy they had investigated years ago, the same guy the neighbor ID'd and described to them since they didn't witness him trespassing at the time -- these are all valid and highly important questions.
As well as interpretation of the statute "within immediate knowledge", "reasonably and probable suspicion", "attempting to flee" a felony which all are not the obvious scenarios for citizen's arrest.
Personally, for the misdemeanor immediate knowledge I imagine something like a woman screeching "stop, he took my bag" and seeing a guy running from the direction of the screech holding a bag. Or a store owner pointing out a specific person who they witnessed shoplifting to a security guard on the scene. This is case is a big stretch from those scenarios, and the arguments can only begin with a presentation of evidence to establish their state of mind at the time which we don't have.
Believing they were legally justified in attempting the arrest is just too far from reasonable in my mind, although I think it is possible they could establish in the evidence something which is worth testing.