Yes a rowing machine can help to flatten your stomach by working your ab muscles and other core muscles in your body.
The rowing machine exercise helps flatten your stomach and benefits your bodies abs by constantly engaging the core throughout every rowing stroke.
A rowing machine also helps to benefit your heart health and improve blood circulation which helps to prevent heart attacks, strokes, blood clots etc.
Your body and leg muscles will also be much stronger as a result of using a rowing machine for exercise.
I used a rowing machine daily for 6 months and after 6 months of rowing on the rowing machine I noticed a much flatter stomach.
So it can take awhile to get the flat stomach or lose belly fat as a result of using a rowing machine but you will eventually see a flat stomach or lose belly fat if you keep up with the rowing machine exercise.