we thought denchura was electric long before we saw him using the electric attack :) his name is electricity + tarantula! and i doubt they'd have him using an attack in a commercial that wasn't his actual type.
as for fireflies, don't take it too hard, but people do call them lightning bugs too XD do you think children care about all the scientific reasons their light isnt actually battery powered?
I know about his name, there's still the possibility of it not being Bug/Electric. As for the attack he uses, Wooguru and Hihidaruma use Superpower, yet they are not Fighting-type. There are many more examples as well. Ever chewed a Wint-O-Green Lifesaver in the dark? It glows, so does that mean it is electricity? I know kids don't care, but their logic isn't necessary for the designs. I'm just going to stop because Lanturn is based on an animal that uses bio-luminescence, and he's part Electric. lolz