Although the retort was correct in its definition of probability, the original premise of the essay was not that probability is immutable, but that the possibility of an event occurring will always exist. The agent's response of "there is always a probability" is an accurate one. The concept of probability is based on data points, which can and do change over time. However, the probability that I would make the flight at any given point in time did indeed exist, and will continue to exist now and forever. So while the agent's response may have been an oversimplification, she accurately conveyed the fact that there will always be some probability—some chance—that I made that flight.
The retort also misses the conclusion of this essay—that despite the fact I didn't make the flight in question, there will always be a probability that I did. There may have been a low probability that I would make the flight by the time I arrived at the airport, but considering all possible points in time during which I was able to make it, there is still some small chance that I could have made it regardless. In other words, after all's said and done, there's actually still a chance that I still UrRonged my way onto that plane!