[TexasFreeper2009]

Simple species X is just the offspring of Homo sapiens who humped Neanderthals.

Heck there are guy humping sheep, you can't tell me they wouldn't hump a Neanderthal girl.

[smokingfrog]

I guess that's what they mean by inter-”action.” Why didn't they just say they were humping each other? Of course, we don't know if it was consensual.

[Caesar Soze]

X-woman branched off from the common ancestor of sapiens and neanderthal 500k years before sapiens and neanderthal split (per BBC). Even if the latter two homos got to humping, their hybrid offspring would be distinct from X-woman.

                  |........................................................ X-woman (48 - 30 KYA)
                 /
Common ancestor |                                          | Homo neanderthalensis (300 - 30 KYA) 
(1 MYA)          \                                        /
(Homo ergaster?)  | Homo heidelbergensis (circa 500 KYA) |
                                                          \
                                                           | Homo sapiens (200 KYA)

Dates are really rough, but it shows how the three species could have co-existed.