No, that's not so unreasonable. It's not true, of course, it's an assumption, but in a highly connected network the broadcasts are quite reliable. You can try to amuse yourself by computing the probability of a message being not received by a peer when everyone is connected to N other peers, as a function of reliability of a connection and size of the network. Imagine that everyone passes the message to all neighbors once upon receiving it.

Now with not-always-online nodes there may be problems, but again these could be addressed to minimize damage to them.