Actually, THIS is the lightest mountain bike in the world - MBR
129g Lightest and Strongest Carbon Fiber Seatpost with Titanium Bolt for Long Seat, Couple Seat, Cafe Seat, Motor Seat, Escooters, Ebikes and MTB, Sports Equipment, Bicycles & Parts, Parts & Accessories on
TLO Carbon Seat Post – Velo-Italia
304g Vertical Helium Dropper Post Claims to be World's Lightest - Bikerumor