A more charitable interpretation would be that parent is replying to grandparent alone, having found the comment on 'new', and is unaware of the great-grandparent comment that prompted it.
The "comments" page (not "new"; that's articles) has no reply buttons, presumably because replying without context is anti-conversation. I don't think it's charitable to assume other commenters would do that.
So there isn't! I occasionally look at that page, but have never gone so far as to actually try to respond to anybody that way, so I never noticed the lack of Reply button.
This is a thread about people being dumb, I suppose, so I am proud to have played my part.
