Pounds are a unit of force, while kilograms are a measure of mass, true. However, on Earth they are interchangeable because the difference between a measure of force and a measure of mass in this case is gravity. The pound takes into account gravity, and mass does not.

However, so long as they are in the same gravitational field, they are effectively the same unit. You can think of it as adding gravity to kilograms to make it a force or removing gravity from the pound to make it a unit of mass. Either way, you end up with comparable results if the gravity is the same.

It also does not matter what gravity it is, on the Moon, Earth, Jupiter, or a neutron star. So long as the gravity for both is the same, kilograms are effectively a unit of force and pounds a unit of mass.