Thought most of my electricity comes from Belews Creek which like Marshall is a coal plant there are a few solar fields within a few miles of my house so some of the electricity is coming from solar.
Even with Marshall and McGuire near Charlotte I would think there is some solar in the area around where many UNCC alumni live and work.
You are correct that electricity is still very much a mixed bag, but an internal combustion gas engine has a Carnot efficancy of around 35% at the upper limit I think., and I think lawn mowers are not as efficient as automobile engines. They are not regulated in the same manner. I think your coal burning steam plants are a bit higher efficiancy than gas engines so getting energy transported over a metal wire to your home is probably a bit better than burning a fossil fuel at your house. Also these battery powered tools are much quiter so the neighbors should appreciate that and they do not get hot so less likely to burn an arm holding a trimmer.
TL;DR You are correct about electricity not being “clean” but it is still a better option if the enviroment is being considered, and I hope that is part of the consideration.