The criterion that Apple uses for the “Design Style” property in “Smart Collections” is undocumented.
However, it’s likely to use the PANOSE data in the font, which is used exactly to describe the visual characteristics of the typeface.
From a quick look at your font, the PANOSE data is set to “Normal irregular”, rather than the expected “Monospaced”.
So that may be the cause of it.
However, I have had problems getting my own (designed) fonts to appear in the “English” collection in Fontbook; but following a OS install and reboot, they are now showing. So it may be a font cache that needs clearing, or just a reboot.
