That's a total load of bull. Children of bilingual families have no issues.
It is actually extremely HARMFUL to not speak as many languages as possible in front of your child. Children do not learn language the way adults do! They do not 'study' it, they 'assimilate' it. Something that is hard for adults.
Thanks to that, a child that learns multiple languages as young'uns will be able to speak all of them flawlessly, understand all of them flawlessly and have easier time learning any other language in the future. They absolutely will NOT mix them up. At least, not past 3'rd or 4'th year of age.
Furthermore, the more languages they learn, the more intelligent they will be as they grow up.
In other words, what is shown in this chapter is how to hurt your childes future for no good reason. The exact opposite should be done, surround him with as many languages as possible. It doesn't matter whether they would be spoken to him or around him. As long as he will hear them, he will assimilate them and grow with them. Proof? My English is learned from watching cartoons in English when I was a kid, then 'leveled up' as I started playing games in English. I've never, not once, had an issue with differentiating between English and my native language, and instead I can think in both. I do not need to translate between them in my mind, ever. My nephews have parents with two nationalities and hear both languages. They also have no issue differentiating between the two languages. Not now, not when they were just beginning to speak.