Well, it still sounds like pure speculation by Serino (if in fact Serino said this.) Honestly, what bothers me more if indeed Serino is speculating this, is that it seems to me that if Martin initiated a confrontation first, Zimmerman’s account of self-defense following an assault by Martin is more credible to me, not less. But his judgment in leaving his car to find Martin is more difficult to explain, given his history of reluctance to personally confront suspicious people in the neighborhood.
Speculation, fabrication, or Z gave testimony to that effect.
I agree with your other remarks; and they mirror Serino's comments to the young witness's mother - the two encounter theory is useful to ascribe sinister motive to Zimmerman.