Why would a german company follow a standard developed by one American company for its european customers? Why will they sell other people's design if they have their own accessories to sell in their own markets? American consumer preference means nothing to HK and their military customers. The only thing that matters is NATO. Only the rail itself is NATO, as to how a rail is attached to the hand guard is whatever people are wiling to sell/buy.
They have already sold the HKey to the MOD SA80A3 upgrade. There are already more HKey actually in use than Mlok in the military.