I would think that is in reference to the temperature inside of a closed outdoor garage in Florida in the summer being akin to being in a solar oven.
One of the best Electonics Engineers I ever worked with one day told me that most chemical reactions happen twice as fast for every increase of 10 degree C ( 18 degree F ) . And while chemistry was not what his doctorate was in, he was unbelievably right about almost anything he did or said. I do not know the exact relation to speeds of chemical processes and suspect that it might be more tied to the distance that the temperatures are from absolute zero. But any way you look at it, the average ambient temperature that the battery is exposed to has a huge effect on how fast the chemistry of it ( think acid ) causes things to wear out.
Take that same battery and put it in an identical vehicle and park it in an underground garage ( something almost unheard of in Florida ), and with only a change in average ambient temperature of being 18 F lower the battery may very well last twice as long.
As another example, batteries in Arizona do not last long.
So, the question about if it was parked in a closed garage is a good one, especially considering that the OP is in Florida.
And if it is kept in an outdoor garage, it might be a good idea to look at how hot it gets in that garage on summer day, and means of reducing that temperature such as ventilations methods.
Also, hot ambient temperature storage of vehicles can degrade the life of tires, rubber seals, hoses, belts, interiors, and probably several other items.