I'm pretty sure they are doing the latter. I'm not aware of any feed readers that don't accept Atom and you will get much more consistent results as it is more clearly specified.
Yes, Atom is fine (preferable, even...at least to me). We're probably stuck with "RSS" as the generic term, though, for much the same reason we still say "making a record" for music or the floppy disk icon for "save".
Any reason(s) to use RSS (0.9x/1.0/2.0) over Atom (RFC 4287)?
* https://en.wikipedia.org/wiki/Web_feed