We are talking about consultants, but CTOs should have this knowledge if they if they are the ones making system architecture decisions. I have been at positions in which I had to explain to hierarchical superiors why a decision was bad or why some architecture was preferable. I was also overruled in some of these occasions, and in these cases we hit obstacles I predicted and had to change on the fly, ended up with delayed deliverables and a sub-optimal solution.
It's not realistic to expect CTOs to know the intricacies of all the latest widgets and libraries. I'm talking about CTOs of mature companies, not "CTOs" of a 10-person startup.
