Agreed that sprinting outside is optimal
Disagree that treadmill is worthless.
Most research indicates about a 15% difference in cals burned on a treadmill due to the fact the track is moving. That can be compensated for with incline. It's not the best, but when the situation dictates--like winter in WI--it's better than doing nothing
So you scale your workout a tad differently. It can still be a safe and efficient workout.