Yes but when you get down to it, almost every activity you perform daily could be considered a detriment in terms of opportunity cost, unless you are progressing towards your goals at every single moment of the day not including sleep.

Now if the author had said, "It never hurts to give up everything else you are doing and look at this codebase for 1 year and shirk all other responsibilities" I'd agree with you, but otherwise you're just being pedantic for the sake of it.