At his time of 2:37, where does the extra battery boost come from?
At first it's accelerating with boost to about 290kph when acceleration slows a lot. I suspect that it's charging the batteries from the MGU-H there (which would mean a little less power available), and once it has cleared the last of the slight lefts on the uphill climb, it has enough energy again to release the boost and launch it to 340kph. (Although it looks to me like the graphics don't show the whole truth of the charge status)
They clearly have given the deployment strategy some thought to optimise its effect though.