If you use the "mono" slider, it adds the angled line inside zero for differentiation while monospacing the characters. Presumably, you'd want monospace characters for programming, so I think that's fair.
Your criticism is true for non-monospacing, though I think any fonts suffer from that.
There are lots of proportional fonts that have easily distinguished 0 vs. O (usually, zero is thinner, though often some of the distinctions used in monospaced fonts are also used in addition to shape differentiation.)
There are people out there who code in variable spaced fonts. Not sure how they can myself but some report feeling more productive that way, and for them this could be more of a consideration.
A lot of the text animation on that site does not seem to work in Firefox.
The three-D cube of letters on top is supposed to be in a range of weights and styles. On Firefox they are all the same. You can still drag the cube around with the mouse but the characters don't change as they do in Chrome.
The font selection drop downs change the font in many of the features on the page, but this doesn't work on Firefox either. I'm trying it the latest developer version.
There are a lot of sliders and pointer-dragging features that immediately change the text characteristics but none of this cool stuff works on Firefox. At the bottom it says "Made by friends of Google Fonts", maybe that's an indication of their browser alignment.
> P.S.: This world really does NOT need any more look-good-only-under-Chrome websites.
Incredibly, the site actually performs better for me in Safari and Firefox, than it does in Chrome. It's easier to drag the box at the top in the first two, but it's supremely laggy in Chrome.
Your criticism is true for non-monospacing, though I think any fonts suffer from that.