Just normal randomness I suppose. I've put that URL at the top now, and included the submitted URL in the top text.

The official blog post was submitted earlier (https://news.ycombinator.com/item?id=46990637), but somehow this story ranked up quickly on the homepage.

@dang will often replace the post url & merge comments

HN guidelines prefer the original source over social posts linking to it.

Agreed - blog post is more appropriate than a twitter post