The font stack is just "Verdana, Geneva, sans-serif;" so either one of those or a system font is the culprit.

It’s Verdana.

Fun fact: the article uses U+03C0 GREEK SMALL LETTER PI (π), but Unicode also has several pi codepoints meant to be used specifically for math. E.g., U+1D6D1 MATHEMATICAL BOLD SMALL PI (let’s see whether HN strips it: [edit: it does, see [0]]).

[0]: https://en.wikipedia.org/wiki/Pi_(letter)#Unicode