With good reed work and a little embossing, any harp can play overblows as passing notes.
The overblows in this video can be hit cleanly. They cannot be played with any more expression such as being sustained or bent. That's the difference between a regular harp that can play overblows and an "Overblow" harp. The overbends on the Overblow harp are meant to be played with expression and will not squeal or drop out when bent up.