To clarify that point ...
Two "supposedly identical" engines might produce different torque (and therefore horsepower) at the same RPM due to inherent production tolerances, or historical wear (piston ring sealing, bearing wear), or different operating conditions (coolant temperature, oil temperature, intake air temperature, fuel quality, and a whole bunch of other factors) BUT if they are the same RPM then the torque and horsepower are mathematically linked, because one is a mathematical construct of the other.