This is often true, but there are exceptions. Last year I followed nostr:npub1tuqwrnzqmu90u45dm29qqfs6xslyg7gs0d5mqsymym9jqwjrupeq2jy8s9 , who seems nice if a little too into politics. Unfortunately whenever I replied to her, she never responded. I think her Mastodon server is forwarding her posts to Nostr, but not Nostr notes to her. (Or she’s ignoring me. 😕) It’s a one-way conversation now.