Samsung officially acknowledges "Mystery Lens", a prototype only, not coming
Amateur Photographer UK got an official quote from Samsung on their 300mm f2.8 mystery lens that was shown in their "Behind the Scenes" YouTube videos. Samsung confirmed this is a prototype, and their exact quote quoted by APUK is "there is no definitive plan to launch this lens into the NX system"
The lens looks more like an SLR lens anyway, most likely a carryover from their GX-series K-mount days. Regardless, a giant 300mm f2.8 lens on a tiny NX mirrorless camera would be about as mismatched as Shaq standing next to Muggsy Bogues ;-)