The problem is that practical advice is often misinterpreted or misconstrued.

"the idea that a person can't eat more while exercising"

You can, of course, eat more while exercising.

But you can't eat much more while exercising, because running while eating is a choking hazard.

I'm kidding, but that is the nature of what I'm getting at.

But really--you can't eat much more during the day because exercising just doesn't burn much more calories. And eating a lot more calories is relatively trivial.