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.
And there are also alternative versions of many letters as shown further down on the page.
Yes I don't understand how they can claim it's optimized for legibility when the base font does the inverse.
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.