Exactly. He's not saying "don't think about optimisation until you've written all your code". He's saying "don't try to optimise until you know where it's neede

Exactly. He's not saying "don't think about optimisation until you've written all your code". He's saying "don't try to optimise until you know where it's needed the most". In many cases that does mean write a minimal implementation first. However in some cases knowledge, experience and early profiling will give you some pointers before that.