Language changes. Words frequently develop the opposite meaning of what they originally had—opposites seem to be semantically closer and more prone to switching than completely unrelated words. When a word changes meaning, it is not wrong to use it in the new way, and at some point it even becomes wrong to use it in the original way: if you used "terrific" to mean "inspiring terror", you would confuse most of your audience!
In this particular case what I find funny is that the author acknowledges that this semantic shift has been going on for hundreds of years and all that was holding it back was the language purists. According to their own account, when the purists fell out of favor in the 60s it was like a dam burst.
The "incorrect" usage recently overtook the correct one in published books:
https://books.google.com/ngrams/graph?content=%22comprised+o...