Thank you for this question. I’ve been waiting for someone to ask 😄
Right off the bat, I don’t believe there is any contradiction between the two statements. If you see two guys in suits down the hall in the office, one in an Armani suit and one in a Budget H&M suit. It’s easy to confuse them from afar. But if you work with them day to day, you’d know.
You see, at a large font-size, I’d say 24pt and up, the difference is very pronounced. Circular has a much stronger personality and it helps you give a very sleek and friendly vibe. Open Sans is a good font, but it’s very generic in the sense that it’s made to look fine in every context, which means that it never looks great anywhere. It’s a big missed opportunity especially for headers and titles.
In a smaller font-size, yes, the differences are more subtle and you can indeed confuse them. However, circular renders so much better meaning that for the same font size, it reads much easier, the glyphs are easier to distinguish and they keep their “personality”. So, even though the user never really sees them side-to-side, you’d have a better experience with circular (in contexts where circular is a good choice)