I don't think the article as advocating pure functional programming. More of a data oriented/array programming style, which has been gaining traction among game programming. There are similarities but more differences than similarities.

I recommend you watch the Mike Acton talk he linked at the end (even if you aren't a C or C++ developer).

P.s. And yes, I agree you should measure (obviously), however that is only one half of the equation. You make assumptions about the performance issues and verify them by measuring. Measuring is very poor signal to noise, but your assumptions can be wrong. Both are needed to optimize code well.

If what you said were true, it wouldn't be possible to design for efficient cache access. And it is.