"programmers well versed in functional programming are extraordinarily productive in any language"
This could just be due to the fact that people who learn functional languages are interested enough in learning languages that they're productive in most of them. If, for example, functional languages were the norm, and some people decided to experiment with OO languages, then it might be that people who knew OO languages would be the ones that were productive in any language.
Except that FP basically dominates class based OO[1]. If FP were the norm, nobody would study class based OO. Remaining alternatives could be prototype based OO, bare-bone programming (assembly, C, LLVM), stack languages, and domain specific languages.
That's an opinion, and it may have merit and be valid, but it still could be the case that people might want to experiment with OO languages, and also doesn't change the fact that people who know FP might just be better in any language because of their interest in programming languages, not because of the merits of FP.
This could just be due to the fact that people who learn functional languages are interested enough in learning languages that they're productive in most of them. If, for example, functional languages were the norm, and some people decided to experiment with OO languages, then it might be that people who knew OO languages would be the ones that were productive in any language.