What I mean is that back then, you were on the same network as everyone else on your _block_, not just your household. You couldn't send packets just to your ISP's gateway, due to the nature of how the coax cable was wired, you're sending packets to everyone else connected to the same ISP node. (Ethernet as a protocol is designed to ignore unicast packets that are not addressed to your MAC address, which is why this works just fine.)

You couldn't connect a simple hub with multiple compuers to your cable modem back then either, IIRC because your MAC address had to be registered to their gateway, and it didn't really tolerate having multiple MAC addresses on the same cable modem. Which is why consumer routers started to get popular, because it allowed you to "share" your cable modem connection to multiple machines in the same home.

Edit: 1000100_1000101 explains the physical setup better than me: https://news.ycombinator.com/item?id=32302361