[jdjb]

The linked article explicitly states that the author has found (albeit anecdotally) that good engineers are very very productive in PHP. I've found this myself as long as you don't go writing code that sits in PHP's "hmm, what'll happen if I do this..." area.

[jronkone]

AFAICT, "good engineers are productive in X" doesn't imply that they wouldn't be yet more productive in Y.

[byroot]

Also the Facebook mantra being "move fast and break things" (that anyone working with their API appreciate), it's easy to have a high productivity in most languages.