HN search (in the page footer) uses the great service at https://hn.algolia.com, which will return those results, and more, indexing post titles, comments, and other categories for search.

Though I imagine he has a script for this use case though.

* "related. comments" https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...

* "related. comments dang" https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...

* "Related. comments dang news.ycombinator.com" https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...