Physicists confirm possible discovery of fifth force of nature

You are correct. At high temperatures, electromagnetism and the weak force merge into the electroweak force. However, these temperatures only existed at the very, very early stages of the universe. As temperatures fell, the two forces split apart into the (rather different) ones we see today, so it is habitual to refer to them as different forces, even though strictly speaking they are different manifestations of one underlying force.

The phenomenon is called "Electroweak Unification", and it basically posits that the forces act indistinguishably under certain very rare (read: highly highly highly energetic) circumstances. They aren't the same force, but simply appear to be "unified" under that regime.