Ok, let me make an example. In the paper the cost is in $, but let's simplify and use time. Let's say that we want a high PoW barrier, like 60s (average) to send an event to Nostr relays using a smartphone. Let's say that the same message costs on a server something like 10s, as the server is more powerful. So the Nostr user will be pissed ok by waiting 60s to send a message, which will also drain his smartphone batteries. On the other hand, the dedicated server of the spammer will send 8640 spam messages per day, flooding Nostr relays. And that's assuming the spammer has a single machine, but in reality could be a srvrfarm. You can reduce the cost and make it even easier for the spammer. For Tor I am not sure. Bitcoin is very different: PoW is a competitive game to sign a block, it is not an antispam. There is no parallel to a messaging system.