If the story is true as reported, Ouslon clearly was not covered by any “stand your ground” law. Getting popcorn thrown at you is not a reason to believe your life or well-being are in danger and does not justify shooting the popcorn thrower. Getting up to confront someone about texting in a movie theater in the first place is an indication that this was some hot-head looking for trouble - texting makes no noise and should bother no one else.
It's not a "stand your ground" law. It's self-defense law.
Technically however, he might have been. Florida law also allows the use of deadly force to prevent a "forcible felony". There is also a law that reclassifies battery against a senior citizen as a felony. So, technically, throwing popcorn at a senior, which is battery, becomes a forcible felony, and deadly force is allowed to prevent.
I'm not saying that's how it should be, or that a jury would buy it, but he does have a shot at a legal defense.