The scientist is incorrect in his statement.
The new seafloor is formed at places where there's divergent plate boundaries. Because the two or more plates are moving away from one another, the crust becomes thinner, so the high temperatures, pressures, and magma manage to penetrate from the mantle very easily. The magma that comes out on the surface is cooling off very quickly and is creating igneous rocks, thus new crust, or new seafloor if you will.
The trenches are appearing at places of convergent plate boundaries, and they are not places where new seafloor is formed, but rather places where one tectonic plate submerges under another tectonic plate.
