Here is the previous Mr. Smarty Plants question to which you are referring. It was published first in the Ask Mr. Smarty Plants section of our website and then reprinted in the Austin American Statesman, which we very much appreciate.
You are correct, southern Florida is not the only place in North America where an avocado will grow. If it were in our Native Plant Database, which it is not because it is not native, we could go to our webpage on the plant and discover which states it would grow in and, again, follow the link to the USDA Plant Profile Map to see if it grew in Travis County.
The original question was "will it grow outside in Travis County?" From GardenGuides.com., here is an article on Hardy Avocado Trees in Austin. As you said, they point out:
"Traditional cultivation of avocado trees (Persea americana) takes place in the lower Rio Grande Valley in the southern tip of Texas around Corpus Christi and Brownsville. Austin is much farther north, and typically gardeners there deal with thin alkaline soil--much more challenging for successful cultivation of the subtropical avocado. Avocado can be grown in containers but inevitably becomes a tree taller than 30 feet. Austin is in U.S. Department of Agriculture Hardiness Zone 8b, although a warm winter may temporarily make it zone 9--cautiously good for avocado culture."
To take this just a step further, Austin, TX is in USDA Hardiness Zone 8b, the southern tip of Florida is in Zone 10b, and Weslaco, in Hidalgo, right down in the southern tip of Texas, is in 9b to 10a. If you notice in the quotation above, the thin alkaline soil of Travis County is also difficult for the avocado. We don't pretend to be right about everything (a tough thing for Mr. Smarty Plants to admit), but we just answer the questions, and our answer was basically, we don't know. We told that customer that if the only choices were keep it inside where it had outgrown its space or move it to Travis County, then there was very little to lose in transplanting it outside in Austin, but we did not guarantee that it would survive our soils and winter temperatures.