two different things. ewc is fertilizer, with a healthy dose of micro-organisms that are good for your soil. they're one of the best organic fertilizers you can use, but do little for the plants immune system.
insect frass is like you getting a flu shot. there's just enough marker chemicals in the frass to make your plant think it's being infested. it kicks in its immune system, makes chemicals that repels pests, and will build thicker cell walls.