I'd say there are a couple of issues at play here.

For censorship resistance, there's two levels of censorship resistance. 1. Censorship a company/platform, and 2. Censorship by the government. Nostr should be resistant to the first kind because no single company controls all the servers. This would be unlike something like Twitter, which controls all its data.

Resistance against government censorship is trickier because you're right that ISPs are a target. In practice, it might be hard for a government to crack down on all nostr/bitcoin traffic without shutting down the whole internet, because they'd need a reliable way of detecting whether an IP is hosting a nostr server and whether encrypted traffic is nostr traffic.

Because of that, when a government wants to censor information it usually goes after platforms like Facebook, Twitter, etc, first, before going after the ISPs. But, as I said, that won't work for nostr so they really would have to go after the ISPs, which woudl probably result in less effective or more costly censorship.