Some of this is probably culture shock. Dan grew up in Russia and immigrated to the UK. Russian engineering material is usually very direct. If you do something stupid, then it will say what you are doing is stupid. There is no thought towards the feelings of whoever is told they're doing something stupid. Western textbooks for better or worse try to not be as scathing and try to gently let the reader know that they are going down the wrong path.
I much prefer the Russian approach. We are professionals doing professional work, our feelings are not part of it and if your ego matters to you that's your own problem. It is objectively stupid to use something named __SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED because, even if it works, you're not meant to use it! And they made it very, very, unmistakably clear. Dan did the right thing by not budging.