As you know from reading the link, electrical signals and light are both EM waves and both travel at the speed of light in the medium they're propagating in. T

As you know from reading the link, electrical signals and light are both EM waves and both travel at the speed of light in the medium they're propagating in. The copper isn't the medium. The EM waves are almost entirely outside the copper. The medium is the insulator (for a cable) or the glass (for an optical fiber). Sorry to repeat all this material from the link, which you've definitely already read.