IP-based Ethernet -> not realtime capable (without extensions)
Python (garbage collected language) -> not realtime capable
You can at most claim to achieve soft-realtime. As in "sub-500ms but only most of the time".. o_O
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.
This of course clashes with the RTOS style use of the word, and we aren't claiming that Matrix has any hard realtime timing guarantees at all, other than the soft realtime behaviour media stacks like WebRTC give you by means of high priority threads.
In retrospect it's a confusing term; we should probably avoid using it :)
disclaimer: use of the word realtime in the Matrix.org blurb is all my fault...