When you just say "delta-v", it means nothing to anyone who doesn't already know what that means, and it provides no additional value to those who do, so the comment has no purpose. At least a complete sentence like, "This illustrates why it's difficult to overcome the large delta-v between Earth and Mercury," would say the same thing in a way that could actually be useful to someone. Even better if it defined delta-v as well.
Right because HN is a tutorial? Nobody here can understand much without detailed explanations.
The purpose is, so folks can look up delta-v and start to understand. It's not necessary to recapitulate everything on the web in a post. For instance I google 'orbital mechanics delta v' and the whole story is right there for the taking.
