Maybe not "don't ever" but, thanks to my health over the years, as well as some of the cargo I've pulled, I don't always have such speed to use. This being emphasised so strongly so early in a cycling manual strongly suggests that the official view is that the likes of me should not be cycling on UK roads!
You have managed to miss the point that being able to go fast sometimes to make your life better is not the same thing at all as having to go fast to make your life tolerable all the time.
And so on.
And things like complaining that a section on hazards other than motor traffic that criticises many other cyclists doesn't mention the specific thing excepted in that section being as bad, it really comes across as one of those instances where it turns out that you can always find a fight if you go looking for one.
Cyclecraft is not perfect by any means, but as a source of useful advice on how to stay not-dead in UK traffic it's a useful resource. With a press date of 2014 it is entirely reasonable in deprecating the farcilities typical in the UK and downplaying the idea they're likely to be preferable to the roads. They are very often not, certainly if time is of the essence on a given ride.