With Firefox's Inspector panel I can see there's a bit of CSS that is applied to any links that changes the font for the link text to Indy Serif. That has a more stylised X.
[EDIT] Ah, it seems it's applied to the entire body, so it's just the X character in the Indy Serif font.
Can someone help me understand what thr difference between the stylised ‘X’ used throughout the indy article is vs the X on my phone's keypad?