He wasn't arrested. The town FAQ published on March 23 explains why he was not arrested.
The act of arresting is a critical formality, accompanied with being Mirandized and being told the reason for being detained against your will.
I do agree that the public was misled in the first place, to imagine that he was questioned at the scene for 20-40 minutes, then released. And I agree that being cuffed and taken to the station for 4-5 hours of interrogation is different from what most people attach to "wasn't arrested."
As for handcuffed without being under arrest, you can find all sorts of discussion here -> Google: handcuffed without arrest
Being handcuffed can be a form of detention without arrest, and it's everyday common.
According to this florida Attorney, he was.
"2. WHEN AM I CONSIDERED "UNDER ARREST"? When the police significantly deprive you of your freedom of movement in order to question you about a crime or take you into custody, you are legally considered under arrest. You could be under arrest even though no one has used the word “arrest.” ............."
I concede it's a minor issue since Zimmerman was clearly trying to accommodate the SPD in any way he could.