This is one of the more bizarre auto-editing of titles it does because of how pointless it feels.
"The man who" is established English convention. "A man who" is changing the meaning for the sake of it without producing a clearer title. The person is unambiguously defined, so the definitive article is entirely correct. If the person isn't one of a set of people to whom the description could apply, it makes no sense to use the indefinite article. If a title was, "The man who starts every day with steak and chips" then switching to, "A man who starts every day with steak and chips" feels correct, he almost surely is not alone in doing so.
In contrast, "A man who coined the word Robot", is not correct, it alters the meaning to suggest that someone else could also have coined the word.
HN also removes "How" from the start of titles which often destroys meaning too. That is explained as trying to "reduce clickbait" titles, but the effect is often to make the title nonsensical.
It's one of the many contradictions about this place.
I feels as if whoever set up this system hasn't truly understood the meaning that "The man who" conveys, and has attempted to force a version they see as "more correct" despite it producing nonsense.