This is easy to understand if you understand relays. Quick primer:

You connect with other ppl on nostr via relays. When I post a note, that note is sent to all the relays I'm connected to. Those relays then sends that note to all the other ppl also connected to those relays.

Ppl can further boost or broadcast my note which then sends it to all the relays they are connected to, sending the note far and wide.

If a relay does not honor the delete request, that note will remain and continue to propagate thru the network.

Similarly if the note gets sent to a relay I'm not connected to, the delete request won't reach those other relays and remain alive on the network. I wouldn't be able to see it since I'm not connected to the relays serving those notes, but I wouldnt see it but others might. That is, until that note makes it's way back around to one of the relays I am connected to.

Therefore delete is possible but not guaranteed.

Hope that helps?