It's 339 bytes of CSS which loads 2.3k of font css which loads 21k of woff2. 23k requiring two round trips (three if the stylesheet is in a separate file, four including the initial HTML load), and to prevent FOUT, the browser won't show any text at all until all the round trips are completed (or it decides it's taking too long and shows the wrong font). Plus you're adding a dependency on Google for no good reason.
I mean, it's not exactly worse than other websites, it's maybe honestly worth it because Fira looks pretty good, it's just not really 339 bytes.
I agree completely regarding light weight fonts. Wikipedia recently changed their mobile view font to Segoe UI Light for body text, and my old eyes can barely read the damn thing.
We're talking about different pages - one is the page with the blog post about this 339 bytes of responsive CSS (it's 2+MB). The other is the demo page with 339 bytes of CSS + the web font. The former is what the parent was talking about.
I mean, it's not exactly worse than other websites, it's maybe honestly worth it because Fira looks pretty good, it's just not really 339 bytes.