You know, I wonder if we need to clarify the type of bike we're riding. If you're upright, on a hybrid or mountain bike, you might want to lean forward, whereas on a road bike, even with your hands on top of the handlebar you're still leaning forward, but your shoulders will be back relative to being on the hoods.

In general, if you lean forward from your hips with a straight back, you use your glutes more.