That's basically right. Remove any one cell or any subgroup from the knightship, and it's no longer a working spaceship -- at least not one that moves the same speed!

There are "optional extras" that can be added or removed from some spaceships. These are called "tagalongs", and aren't considered to be part of the ship. That can get a little complicated, too, though, like with another recent-ish discovery: http://conwaylife.com/wiki/Fireship .