That doesn't make sense because 1 horse has a maximum power output of approximately 15 horsepower.
Well, it's scientific in the same way that an accused witch and a duck look the same weight on the scale then. Besides, after a few minutes sous vide, I wouldn't be able to pull with all my usual strength, would you?
Not in a vacuum.
What if it's a spherical horse?
Peak horsepower is not that relevant to the sorts of applications that horses were performing that they proposed to use engines for. In that case they wanted sustained power, and a horse can only produce about 1 horsepower sustained.
1 HP was meant to represent the average amount of power a horse could sustain for a work shift. A horse can’t sustain 15 HP for more than a few seconds at a time.