It looks like Apple might be using the Panose proportion categories, OS/2 Font styles and types to determine a Design style. I think the “monospaced” classification comes down to the proportion being set as “Monospaced”.
Taking Courier New as a common example and using Font Forge to examine the font file:
Whereas iAWriter’s Mono (static & regular) font does not have that meta data specified.
I couldn’t find any technical details from Apple on this so the above is a bit of a piecing things together from reading as much info as I could find about the sorts of ways fonts get categorised and what meta data they have.
I did try using Font Forge to amend the file properties, but it threw up lots of issues when trying to regenerate the TTF file. It may be Font Forge isn’t up to the job, I got the save settings wrong, I was wrong about the particular source for the Monospaced classification, or I’m, just entirely wrong about the whole thing; but for whatever reason, even after amendment and reinstalling Font Book didn’t recognise it as monospaced, and of course, neither did Drafts.