I think the single sink is to give some counter space in a very small galley kitchen.
We buy plastic bowls to wash in so as to conserve water use, and our small grey tanks.
One could use the sink, minus a dishpan of some sort, but water usage would differ as would one's ability to choose where the dishwater goes.
There is adequate cooking space in a small kitchen, if you just plan adequately. I have prepared hundreds of meals in mine, for years for a carnivore husband.
The small space is a choice, and some of us prefer it.