Software engineers tend to think in deterministic terms. My code tells the machine exactly what to do and that's it. In real world systems there is uncertainty which requires different thinking. A real-time process needs both, a deadline and the concept of errors. A process must be completed within a deadline or there is an error. As the author said, the absolute time (or latency) is irrelevant. Think of a tennis match. you either hit the ball or you lose. There is a deadline and an error. It's not that complicated yet so many people are confused by the concept.
I've seen real time to mean "must finish in x seconds <i>or else fail</i>" which means technically after x seconds you may simply exit the process, and the definition has been fulfilled.
Real-Time. Consistency. High-Availability. Concurrency. Words with too many different meanings are not sufficient for describing softwares. I treat those words as marketing terms.
The author claims real time has nothing to do with response time. While response time is precisely the point of real time. If a system guarantees the response time of some action is always below a threshold it's real time.