[First, I'm gonna call the new mount EF-R (though it could be RF, or Henry, or In-Your-Face-Nikon-Z)]*
Could we be over-thinking(wishing) this a little?
I'm not convinced the "M" in M.ADAP has anything to do with EOS M mount. It's possible that it's just an EF to EF-R adapter that goes with the new mount, just like the EF to EF-M adapter when the EOS M was released. Perhaps the "sexiness" that was rumored is just the built-in filters.
If the EF-R mount lenses do protrude into the body, thus allowing an EF lens to attach to an EOS R body without adapter, then perhaps the adapter in this rumor is for attaching EF-R lenses to an EF mount camera?
It's interesting to note that the rumor includes super-telephoto lenses with EF mount. That would seem to indicate that EF is here to stay for a while -- at least for pros. I think that points to either A) EF lenses attaching natively to the EOS R body, or B) EF-R lenses being adaptable to future EF mount DLSRs. I'm still leaning toward a regular old EF to EF-R adapter so people can use their EF lenses on the new EF-R mount.
Whatever it turns out to be, I'm looking forward to the announcement.
*If nothing else, Canon won't have to start the "XXX Million EF Lenses Produced" tally over again.
EDIT: Okay, it's probably called RF, if Nokishita is rarely wrong, and that's what they're posting. It's gonna suck to have to start that lens production volume number over again, though...