Friction is the enemy, so why DFCO and have all that drivetrain friction going on? It creates un-necessary friction & heat loss in the transmission & clutch (in manual cars).
So why not put it in neutral and let the car free fall so to speak?
Coasting in neutral has massively improved my FE since I started using it.
Yes when the car is on tickover it uses tiny amount of fuel, but it more than offsets the energy absorbed by the drivetrain on DFCO.
Maybe DFCO works up to 40MPH, but then engine braking kills any benefits and its over to CIN (coast in neutral).
I only wish I'd employed it a long time ago
well you live and learn!