Late to reply, but here's my take. Aggressive deceleration is bad since it involves taking kinetic energy and converting it to store in the battery or bleed of via friction brakes. Energy is always lost during this process. The ECO Drive app is a toy that shows how to combat it. It's not really necessary since the power gauge does the same thing. After a while, one gets the idea of how things work and can do it by feel.I know that on the power side you want to stay within the blue segment for fuel efficiency purposes. The owner's manual does indicate that aggressive deceleration is bad for fuel efficiency and you can visually see it with the ECO Drive app/gauge. I assume if you do get to the bottom(or close to?) of the charge segment it means the friction brake(or some?) is being used instead of regenerative braking hence it's perceived as bad for fuel efficiency.
The most economical (and unfeasible) area of the gauge for the best efficiency is the white line on the power band. There's no loss of kinetic conversion and no use of battery at that point. Staying as close as possible to that line will give the best results.
You do bring up a good question - exactly WHEN do the friction brakes kick in? Honda did such a good job, that we can't even feel when this happens. I know they work at low speed (<15mph?), but when do they kick in under aggressive braking - is it related to power gauge position?