You are absolutely correct in stating that the book is wrong. And just to clarify, U.S. Spiders were fitted with the Motronic Fuel Management ECU starting with the models titled as 1990 (not 1989). 1990 is one of those Alfa Spider "transition" years, so those Spiders have the body of a series 3, but the mechanics that was to be used in the series 4 ('91 - '94).
As to why the information is wrong in the book, I can only say that it doesn't surprise me and I don't think that it should be viewed as a concern about the author's knowledge. Let me explain. There are several factors that should be considered. First, the book was published after Pat's death. Furthermore, and this would apply to all human beings, the amount of information covering so many years is simply of encyclopedic proportions, so it is a daunting task to get everything right.
I know that I have lived and breathed Spiders for several decades and feel confident about my knowledge, but it is not unusual that, as I write on this BB, I catch myself looking at what I am about to post and say to myself: "sheesh,.. what's wrong with your head? this is just wrong and you know better!" It just happens, which is why before publishing a book there is a proofreading process, but of course proofreading technical data is even more error prone since the number of qualified proofreaders is significantly less. I have Pat's books, but to be honest, I haven't read them. In fact, I bought them because I was embarrassed that as a serious Alfista, I didn't own the original classic (the "Bible") or the "Twin Cam Companion", which I also view as a tribute and memory of a great writer and Alfista. When I do get around to reading them, it will be for pure enjoyment. I realize that many people refer to these publications seeking guidance and answers related to their cars, but one just has to realize that publications have errors...
Best regards,