That article contains a misconception about how the MGUH and MGUK interact:
Adding the 700bhp Mercedes is reputed to develop from its ICE, to the regulated 160bhp of MGU-K power and the extra 'free' electrical energy from the MGU-H of as much as 30-40bhp, gives a total Mercedes power output in the region of 890-900bhp.
The MGUK is limited to 120kW (160hp) output at all times. So if the Mercedes ICE is making 700hp, the most it can make, with or without the MGUH delivering to the MGUK, is 860hp.
What the unlimited energy flow from MGUH to MGUK allows is the reduction of energy needed to be extracted from the energy store at any one time, meaning that the stored energy lasts longer during a lap.
The energy flow chart from the regulations: