I'd avoid it for anything important, but if you can read the first ten characters of each line I think you get the point of my list there. Didn't want to waste the space with paragraph breaks between items.
Code blocks are crappy for small screens, but so are lists written as a series of paragraphs.
Unfortunately, HN seems opposed to supporting non-paragraph linebreaks, lists, blockquotes, or any other sort of formatting. That's fine, it's up to dang and the other admins who run the place and they don't want to support other formatting, but it means that the code format gets abused as a poor approximation for these things.
EDIT: I should also point out that your source says "fixed-width is intended for code." You left that word out and it's an important one. If it said "is only for code" the meaning would be different.
Side note, reddit lets you do a line break by ending a line with two spaces before hitting enter. Just tested to double check, it doesn't work here. I wish it did, and replacing space-space-newline with <br> seems like a pretty trivial and unobjectionable feature, but here we are.
In terms of usability, vertical space almost doesn't matter anymore. The vast majority of people have an interface that makes vertical scrolling effortless, whether it's a trackpad gesture, mouse scroll wheel, or touchscreen drag / flick.
Horizontal scrolling is harder to do and less common/conventional (chicken and egg as to why, but it's the reality today).
You might not like the way it looks, but it's clearly better for usability to spend more vertical space in order to prevent horizontal scrolling.
