This idea is important when trying to finish an actual product of some kind, but when kids (or adults!) are learning, let them fiddle with the loops! Let them learn some intricacies and encourage the curiosity.
No one is saying that. Knuth was frustrated with people wasting time on micro optimizations. Those don't really exist in the same form any more. Architecture is far more important to the speed of software and that does need to be dealt with up front.
The problem is when people YOLO their way through a program thinking optimization is for suckers because of an outdated quote from a different context.
I was objecting to the idea that premature optimization isn't a real problem anymore because technology. That's just not true.