Not sure about "nostr is the identity", but one's "nostr identity" is his or her nsec private key. To my thinking, it doesn't matter running your own node or not, it's the protection of the key that matters. Also, not sure how that's backing it up. Writing this, maybe I'm looking at another thing: I was thinking my "nostr identity", my credentials. I think you're talking about creating a backup of my posts and activity and by housing it myself, I'll always have it (unless I wipe it, then it's on me!).