That's not entirely true. Your statement is only completely accurate if your netmask is 255.255.255.0 (or /24), which is quite common, but certainly not the only possibility. For example, the netmask on my home network is 255.255.255.128 (or /25), which means that 192.168.2.1 and 192.168.2.129 are not on the same subnet. Conversely, if my netmask were 255.255.0.0 (or /16), then 192.168.2.1 and 192.168.199.20 (for example) would be on the same subnet. Given that he's using the link-local addresses, I believe that his netmask would actually be 255.255.0.0, so he's good to go on that point, as long as all of the nodes understand this properly.