Good sleuthing! We should fix the error message at minimum.
"body": "{\"deleted\":false,\"message\":\"User XXX has 0 posts, so they can't be deleted.\"}"
It should say
User has x post(s), so they can't be deleted.
We should also probably hint at the situation since this has come up a fair bit
User has x posts(s), either public posts or personal messages, so they can't be deleted