I was talking about page 10 of the PDF [1], in Chapter 1 (odd, the original link in this HN post is now taken down).

The part starting: "The rays of the Sun falling on the Earth can be considered parallel because..."

As another commenter pointed out, the following paragraphs do mention the penumbra and solar disk, but I feel like the reasoning put forth in that initial paragraph is simply wrong (though common), even if it's briefly expanded upon afterwards. Or maybe it's just an artifact of the translation that it comes across sounding more authoritative than it is.

[1] https://gitlab.com/mirtitles/perelman-geometry/-/raw/master/...