Hi! Great question. That's an interesting one because the idea is that regardless of the mass, an object will fall at the same rate.

However, I suppose you could say that is a generalisation/approximation for objects that have a mass that is very small relative to the Earth.

So in your example: yes, the basketball with the mass of the moon will still accelerate towards the Earth at the same rate of 9.81 ms^-2. But, given the mass of this basketball is not small relative to the Earth, the Earth will also accelerate at a non-negligible rate towards the basketball. From an observer which is at rest with respect to the Earth, this will result in a net acceleration that appears higher than 9.81 ms^-2.

The key part is the frame of reference of the observer. If this observer was looking at the basketball, and the Earth from outer space (where they are not greatly influenced by the gravitational fields), then they would see the basketball still accelerates at 9.81 ms^-2.

In summary, what you are describing only changes the observed acceleration from the Earth/basketball's reference frame.