Indeed. You might share a subnet with other customers of your ISP, but your ISP probably doesn't let you ARP poison them.

This isn't even that recent. I remember doing ARP poisoning exactly once and then being surprised that it didn't work everywhere.

I think the falsehood here is that same IPv4 subnet = same link layer network, but that doesn't have to be the case.

Did arp poisoning in school and used a tool to show images from websites people visited on the projector. Teacher thought it was fun until someone went to goatse.