That's because it's actually a Quattrospaced font! Whereas monospaced fonts have a single character width, and Duospaced fonts have two, iA Writer Quattro has four character widths.
This saves space on mobile, and retains the comfortable reading experience we're used to from proportional fonts. At the same time, the monospaced punctuation and relatively few character widths give it that monospaced vibe. 🤓