I'm halfway through reading this and, and so far, the main argument I'm seeing amounts to this: the use of visual art and attention to design is increasing. Making use of artistry in your products or caring about attractive design is infantilization, since kids also happen to potentially find the art and design nice looking because it's known that kids like pictures and color (and adults should like something else?). He mentions that making things less offputting to use is infantilization.

Does this apply to cars as well? Does adding design elements and any sort of color for reasons other than making it more aerodynamic or road safe equate to infantilization? Does using an automatic over a manual? Does using an auto-starting engine over a wind up engine? Does using a car over your own two legs?

I also find the argument that flat design is childish bizarre. I hate flat design, but it's not any more or less childish than what preceded it. Things can actually be more abstract with heavy reliance on text-only elements and icons without immediately apparent meaning, but the author seems to think that's infantilization, and what preceded was also meant for children, and absolutely anything other than what the status quo was in some undefined point in time is too infantilized.

It seems more that the author has a resistance to change, and framed it as everything new being too immature.