The article does not use the Unicode subscript 2 character. It uses <sub>2</sub>. Since the HN post was certainly done by copying the text of the article title, the <sub> tag was not maintained so the 2 reverts to a normal 2.
Someone could have added an actual Unicode ₂ in the entry, but that actually would not be the same characters as the title.
Which is closer to the original text, CO2 or CO₂ ?
