Obviously there's lots of research to be done on the best ways to design programming languages and the best ways to teach them.
However, the headline presumes there can be some universal standard of measuring what we can "usability" that's somehow separate from the practical "usefulness" of the tool itself. Tools that do fewer things can be made easier to use that tools that do many things. Turing-complete programming languages can be made to do anything the computer can do, which is a lot. And so there's going to be a hard limit on how "usable" they can be. Not to say we're even close to reaching that limit, but you'll never make a programming language that's as "usable" as a screwdriver, and that's okay.
Well, I'll admit that a rather debatable claim. But article provides reasonably good backup for that claim using the tools of professional user interface design, a field that has become a lot more systematic in recent years. Towards the end of the article, he describes a number of metric which can be used in measurement of programming languages as task-accomplishment tools.
However, the headline presumes there can be some universal standard of measuring what we can "usability" that's somehow separate from the practical "usefulness" of the tool itself. Tools that do fewer things can be made easier to use that tools that do many things. Turing-complete programming languages can be made to do anything the computer can do, which is a lot. And so there's going to be a hard limit on how "usable" they can be. Not to say we're even close to reaching that limit, but you'll never make a programming language that's as "usable" as a screwdriver, and that's okay.