You might have to ask Fulcrum but seems to me to be a possible weight saving tactic where they have the steel where it's needed and not where it isn't and still functions properly. Cogs will slide on the same as any other conventional freehub splines. Fulcrum HG 11v Racing Light 2014 Freehub Body | Bikeinn (tradeinn.com)

Looks like FBinNY beat me to it above.