So it isn't that those things aren't important; just that they are subjective
What those features are, rather than subjective, it's contextual, and I feel that behind the author's phrase might lie the usual complain by functional programmers that non-functional languages just feel foreign because people are usually taught other styles during their learning process.
As an explanation, that might be partly true, but it being true doesn't really save you from the fact that if you want people to use your tool, you have to make it easily usable, and usable for real world people rather than for spherical cows.
Do we have any objective metrics? The only thing I can think of is all the (inconclusive) studies about error density. Unfortunately error density is a very far cry from measuring the overall quality of a software product, or the mental effort that was required to produce it, or the mental effort required to maintain it.
> there are thousands of people whose job is precisely optimising usability for many products
Yeah but programming languages are more abstract; they're tools of thought (just like mathematical objects), and optimizing their use is nothing like observing a user press the buttons on a machine to get themselves a cup of coffee.
Controlled studies are very difficult to run, for obvious reasons. (Unlike for the coffee machine)