to me, it is more helpful to think of them as two seperate instruments, like the accordion is different from piano, but it's also different like a sax is different from a guitar.
When you play piano, the way you play, what you play is different, you're paying attention to the volume of each note, you have that sustain pedal, you have the piano sound that fades out.
When you're playing organ, you're less concerned about the velocity of each note, you actually have way less resisitance(this can make it harder for beginning organists to get used to), you have this note that will continue to play as long as the electricity bill is paid and you have the drawbars, the Leslie, and the pedals to play, and you're playing from a different perspective.
The roles that a piano and an organ have are completely different, though complimentary.
that's why I think of them as 2 different instruments.