Just guessing here, but he assumes that the thermal energy of that gramme traveling at 7 miles/second needs to be applied instantaneously or nearly-instantaneou

Just guessing here, but he assumes that the thermal energy of that gramme traveling at 7 miles/second needs to be applied instantaneously or nearly-instantaneously, when it can instead be given to that gramme over the course of several minutes via, say, a Saturn V rocket system.