do we know that there is a "real" gomu gomu no mi? (we don't know right? that's my point)
for me it sounds like they just gave that fruit its proper description name: "the rubber fruit"
and thus they "erased" the name nika from the history/devilfruit books
[another possibility: they just keep the paramecia fruit locked away for eternity]
The Gorosei said that the gomu gomu no mi had another name, they didn't talk about a real name. At this point to there's absolutely no reason to believe that Luffy doesn't have the gomu gomu no mi, the Gorosei just revealed that there was a hidden side to it.