It's a more efficient engine, with better particulate filters, a catalytic converter to reduce NOx emissions, and yearly emissions inspections to ensure all of the above is up to standards. It's a gain.
An F150 consumes ~2 litres of fuel an hour at idle and provides 400 watts of power through its outlet. A Bosch leaf blower is 2400 watt. Petrol powered leaf blowers do not use 12 liters of fuel an hour.
Sure, the emissions in some ways will be lower, but it's not an unambiguous gain and it's not anything close to an environmentally friendly way to run a leaf blower.
This is clearly a niche for PHEVs or BEVs.
Where'd you get this number? AFAICT this is not true.
https://www.google.com/search?q=how+much+gas+per+hour+idling...
More like a little less than 1 liter for a 4.6L V8 motor.
(And yes it would, and that's great!)