Sure there is, it's a different thing. Many monospaced fonts are also uniwidth, sure. The article is promoting fonts that are non-monospace but are uniwidth. It would be wrong to apply the term "monospace" to these fonts, so a second term certainly is needed.

All (good) monospaced fonts are uniwidth, not all uniwidth fonts are monospaced.

Monospaced is important if you want multiple sequences of the same length to line up, uniwidth is less restrictive of the font (because "l" and "w" can be different widths) but allows you to bold without changing the visual length.

It really is right there in the article which you don't seem to have read in full and explained fairly indepth what the author means by uniwidth and why it's important to him and possibly to other UX designers.

You have misunderstood the important difference between uniwidth fonts and monospaced fonts. Uniwidth fonts are proportionally spaced; monospaced fonts are not (by definition).

It's a different property

except that uniwidth describes a different thing?