Do you consider add(a,multiply(b,c)) certainly more readable than a+b*c?

The article shows some nice k idioms that I find much more readable written in a concise form. For example, I find |^... easier than reverse(sort_ascending(...)), or <>... easier than grade_down(grade_up(...)). I guess the solution in other languages would be to define more functions for all these common operations, but then you will need much more documentation for all these extra functions and learn them, so I don't see it as an improvement.

At difference of most golfing languages, the set of k primitives is very small. Many people who criticize k forget that property, but it's a very important one. It would be interesting to see the R version and compare the amount of documentation you need to read to completely understand what each of the programs is doing (ie. to read the program). That would be, in my opinion, a better measurement of readability than a subjective opinion (unfortunately, the k9 documentation is too scarce at this point to make this exercise in a proper way).