Different contexts. For weapon systems and industrial systems, real-time operating systems are needed to guarantee true real-time behavior.

That usage makes no sense in the context of internet messaging for the reasons you describe. For human-to-human messaging, I think they're referring to a property more akin to synchronous (versus asynchronous) messaging, i.e. chat versus email. I speculate that the matrix team thinks this is an acceptable overloading because, as a practical matter, the two concepts have nothing to do with one another.