it was derived from eye charts that are explicitly designed to find defects in your eyes - it is not a coincidence the O and C look similar.
Yes I don't understand how they can claim it's optimized for legibility when the base font does the inverse.
And there are also alternative versions of many letters as shown further down on the page.
There's an alternative, more squarish C; about half the letters have alternative glyphs. If you're using macOS Font Book, scroll down to the bottom of the repertoire to see the alt glyphs.