Comment by bonoboTP

Some of this can be taught, some not.

But anyway, my main point was, simply sending them a link like this will be perceived as baffling and rude, and I doubt that it can have a positive causal effect because it's not merely that they don't know about this rule of how to write messages, but that they require handholding. You can ask why they got hired then, but sometimes people can be confident and charming and that's often enough especially in non-programming interviews, or there might be also other reasons.