Interesting question. I've seen collections of audio content claiming to be podcasts that don't have RSS feeds, so the term is definitely used that way. Whether that makes it a valid definition is open to interpretation, the word is pretty new still. Merriam-Webster seems to think that an automated download mechanism is required, though it doesn't specify RSS in particular:

>a program (as of music or talk) made available in digital format for automatic download over the Internet

I can see arguing that this doesn't even rule out walled-garden streams with an auto-play feature, since streaming and downloading are functionally the same (the only difference being what type of memory the data is stored in, and swap memory destroying that distinction entirely).