This prototype is likely just some sort of McLaren test mule since the company has said the first hybrid cars (besides the Speedtail) are meant to be from McLaren's Sport Series, and the 720S is part of the higher-end Super Series. McLaren also hasn't spelled out what powertrains its hybrid Super Series cars will be getting down the road, so any guessing as to what a 720S replacement would receive is still just speculation.
As for what this powertrain is, we're left to speculate. The company has revealed one hybrid powertrain for the Speedtail. It uses a twin-turbo V8 and makes a combined output of 1035 horsepower and 848 pound-feet of torque. A detuned version of that powertrain could make sense, especially as McLaren already uses variations of its twin-turbo 3.8-liter V8 in Sports and Super Series internal combustion cars. Rumors have also swirled about a potential new turbocharged V6 combined with an electric powertrain. A V6 hybrid power plant may make less power than a detuned V8 hybrid, but it's still bound to be plenty potent. All-wheel drive could be in the cards, too, as Flewitt has let on that McLaren is considering such a system with the front wheels being driven via the electric motors.
The newest concrete news from McLaren will be coming out tomorrow with the release of the McLaren Grand Tourer. This car is meant to be more comfortable and livable than the rest of the Sports Series is now, so check back for all the details on that.