No, it wasn't. Officers are allowed to detain a person while conducting an initial investigation. Zimmerman was not formally arrested the night of the shooting but was legally detained in handcuffs.
A person who uses force as permitted in s. 776.012, s. 776.013, or s. 776.031 is justified in using such force and is immune from criminal prosecution and civil action for the use of such force... As used in this subsection, the term “criminal prosecution” includes arresting, detaining in custody, and charging or prosecuting the defendant.
2) A law enforcement agency may use standard procedures for investigating the use of force as described in subsection (1), but the agency may not arrest the person for using force unless it determines that there is probable cause that the force that was used was unlawful.
It's probably not worth arguing, since there's nothing there that hurts Zimmerman and a whole lot of consistency in the retelling, but Florida does not allow "investigative detention" in self-defense cases absent probable cause that the use of force is illegal.