I suspect those who accuse Gardner of being arrogant, egotistical, condescending, etc., ad nauseum are likely to be the sort who either have an ideological agenda of their own, or prefer their egalitarianism served with a thoroughgoing relativism. Gardner was never soft-spoken in stating his opinions, and he had the good sense not to qualify every opinion with "that's just my opinion," or "ymmv," or "but what do I know."
I find Gardner's opinions welcome and often enlightening. His allusions to numberless works which I have yet to read -- and some I only learned about from him -- are an inspiring challenge to read more. The fact that he doesn't water down his message to spare the ego of his less experienced readers is a sign of respect for them -- they can take it, because the alternative is a vapid mediocrity. And if they can't take it, well, they're free to write scathing reviews castigating the hubris of a dead man.
The theoretical and practical concepts and techniques discussed in the book are reasonably thorough and engaging. It is clear that his purpose is to challenge would-be writers to aim for the very best, while making clear -- for those actually paying attention -- that good-enough-for-publication writing is achievable by most anyone willing to put in the time. The implicit idea here is that if you want to be a writer, you will have to put in the time anyway, so why not aim to be a very good writer? I fail to see how this is anything but an inspiring affirmation to anyone who really wants to write.