In such a case, I would consider an editing blunder. Can you give the moves that lead to that position ? I would not be surprised, for instance, if the comment was written with respect to the same line where ...Bb7 is played instead of ...Bb6.
Writing a book is a long process, you study a variation, finally the variation won't enter the book because you found an improvement earlier; you analyse a position, then another very similar but not identical, you check one with the software but maybe you forgot the check the second, then you have to send a draft about a chapter to your editor, meanwhile you are wirting the next chapter, and the ditor wants you to reduce the materiel by one fourth, so you come back to your text, cut this and rephrase that, and at the end you a few days to proof-read the final draft before it goes to print... Even for the most organized writers, errors will happen at some point.
Even without a computer, there is no doubt McDonald would find axb5!+- in less than twenty second if given the diagram. That's why I think this error is due to the fabrication process rather than the chess process. It could be anything, from the comment meant for another section copied-pasted at the wrong spot, to the typo Bb6/Bb7, or the wrong draft section sent to print.
It is sure pretty annoying for you reader when you meet such a mistake - at least, it keeps your 'scientific skeptical awareness' awake when reading. But the author will be the most pissed off by such an accident: not only the message he wanted to pass about a variation is spoiled, moreover his name is asssociated to a 'ridiculous' assessment.