We are in Toronto, Canada. The French "immersion" stream in our public school system is full of kids whose parents think they are too smart for the regular stream but they haven't tested into gifted. It's generally acknowledged as a way of providing extra challenge if the parents think their kids will be bored at school.
However, anyone I know that has graduated from the immersion stream admits that it did them no good - there is a real dearth of teachers who meet all 3 requirements of a good immersion teacher (especially in the higher levels): they know their subject material, they are natively fluent in French, and they are good teachers. So the farther you go in the immersion stream, the worse the teaching gets and the less the kids learn.
My husband was researching this and came across the term "interlanguage fossilization" which describes one result - kids learning a second language from each other and from non-native speakers, who end up creating a language that is not a natural language spoken by anyone.
On top of that, the reasons for parents choosing the French has the further effect of devaluing the regular English-stream, creating all kinds of problems in the student populations of schools where both streams are offered.
I haven't read the book you mentioned, but if it disparages the Canadian immersion system, I wouldn't be surprised.
Fluenz & lefty book recommendation