Inter is the wrong typeface to use here. Safari has a built-in typeface keyword which can be used to invoke the apple system typeface: -apple-system. On Safari & I believe firefox this will translate to whatever the apple system typeface is, even if they decide to change it away from San Francisco. On Chrome the same feature is called BlinkMacSystemFont. I suggest modifying the font-family list to instead include these two, ahead of San Francisco, then fallback to Helvetica if none of those three are available. See https://systemfontstack.com/ for more information, although this is not quite the same thing that we're going for.

The actual code already uses -apple-system.

From globals.css:

body {
  …
  font-family: -apple-system, BlinkMacSystemFont, Segoe UI, Roboto, Oxygen,
    Ubuntu, Cantarell, Fira Sans, Droid Sans, Helvetica Neue, sans-serif;
}

I'm not familiar with the javascript at work here but the typeface being used is pretty clearly not the system typeface. I have no clue why though as reading the style sheet tells me exactly what you've posted. The safari inspector however, tells us the real typeface is in fact SF Pro followed by Helvetica Neue.