You can choose to send them to Nostr but yes every Boost or Boostagram is sent to Nostr. The original post you replied to was a Boostagram from Fountain that I tagged your Nostr profile in.

Only Boost from accounts that have Nostr set up in Fountain are sent to Nostr. With things like the Mostr Bridge I've seen my Boost also show up on the FediVerse.