They may have small teeth as evidenced by the torn up chunks of food that is left in the tank after she has had her fill.I've never seen her open her mouth and none stick out her closed mouth.
People should not compare featherback type knifefish (Notopteridae) to nakedback type knifefish (Apteronotidae) as they are not even closely related. That is is like comparing a great white shark to a black shark which is a labeo.