That’s a good point. Something close to a "properly fitted proportional version of iA Quattro" might be Nitti Grotesk. https://www.boldmonday.com/typeface/nitti-grotesk/
You can't use Nitti Grotesk, a cookie batter, to make an argument here either for a fair comparison. It's an entirely different thing.
In order to do a proper comparison, they should finish the kerning job and present their arguments fairly and squarly.
The image I am talking about is: https://i0.wp.com/blakewatson.com/wp/wp-content/uploads/2022...
It needs a 3rd line "iA Writer Proportional". Looking at the word "next" in iA Quattro, the fitting looks really bad which is a known trade-off in monospaced typefaces, but not an excuse to leave it like that in a proportional typeface.
The word "chain" looks more like "cha in" which just makes the font seem unfinished to me.