I don't see it as harmless at all. It promotes belief in other pseudoscience, like various kinds of alternative medicine. People literally die from using fake medicine instead of real medicine. It's also a scam, and I hate scams.
You're also missing that the astrology book itself is where the idea came from that the exact time of a person's birth is important to making accurate predictions. If anything, your commentary about the time being irrelevant is an argument not just against astrology in general, but against that author in particular.