As I understand it, they don't have to assert that at all for it to be in effect.
Well, define what you mean by "in effect"?
As I understand it, and I'm in Florida, so I've been forced to learn, you both may be right about stand your ground. Sorta.
A stand your ground hearing is held to determine if a case applies, and if it does, there IS no criminal trial. It's not something that effects a criminal trial in and of itself, by which I mean, there's no such thing as a "not guilty by reason of stand your ground law.". So, that's what Hab is talking about: they chose to pass on that hearing, which means they went to criminal trial.
Now, where he's mistaken, and your point comes in, is that nobody "waived" anything. The Zimmerman defense team pointed out that, while it is the usual practice to have that hearing pre-trial, nothing in the law specifically REQUIRES it to be held pretrial, so they can still request that motion anytime they want, like say, after the prosecution rests their case, etc.
At least, that's how the local media talking heads and legal experts have been explaining it.