Ethernet has been around since the mid-70s, but if you think it was always Cat5 and 10BaseT, you’d be sorely mistaken. The first ethernet was built with coaxial cable, vampire taps, AUI adapters ...

But those old AUI adapters were all 10 megabit Ethernet or Thin Net or ARCNet. A hack like this can’t tell you if the line is 100 megabit or gigabit.