Draw a graph of atmospheric pressure over altitude. Is the line straight or curved?
It's curved. Ergo, expending all your thrust at launch is not the most efficient way to gain altitude.
According to that graph, at 1km up (Which is how high the vehicle got), there is ~2.3% less pressure than at sea level.
"Using a lower acceleration means that you're wasting impulse lifting the remaining mass of your fuel for a longer time, hence you'll get a lower altitude. Blasting it out all at once means that more of that potential energy stored as pressure is used to push the vehicle alone, getting you to a higher altitude."
If you expend 100% of your fuel in the first 1% of altitude, it means that you have to travel the remaining 99% (Through the least efficient part of the graph) with no power at all, and that you have way less inertia. So expending all the fuel at the start is not all good - there are up-sides and down-sides.