  While you are correct that 'toughened glass' could be considered a misnomer (toughness has a specific meaning with regard to the strength of materials), tempering glass is not.  In fact there are two ways to temper glass to increase its strength: thermal and chemical.  Regarding thermal tempering, you are incorrect that the surface of a glass is in tension even at room temperature.  On initial cooling you are correct that the surface is in tension and the interior is in compression. However, below the glass transition temperature the stresses reverse and the surface is in compression and the interior is in tension increasing the strength of the glass.  Prince Rupert's drops are a laboratory curiousity demonstrating this phenomena.  If a drop of molten glass is cast into a container of water, it solidifies into a shape roughly resembling a tadpole.  You can bang on the big end with a hammer and the glass drop will not break.  However if you nip the tail, the drop disintegrates into dust usually with a pop.  Thermal tempering is usually applied to windshields.  Chemical tempering is usually accomplished by ion exchanging a smaller atom for a larger atom at the surface again putting the surface in compression. Although glazing and surface crystallization can also be used.  For example, soda-lime-silica glass can be placed into molten potassium nitrate and exchange potassium for sodium in the surface increasing the strength of the glass.  Thermal tempering can increase the strength of glass from 70 to 200 MPa while chemical tempering can increase it 500 to 700 MPa.  Chemical tempering is commonly used to increase the strength of beverage containers. 