Earlier quoted context omitted.
I generally agree. The secondary (and more interesting) question is whether placebo effect is applicable to infants. Based on the theory of how placebo works, it seems it might not be. It seems there are no studies on that. At least I couldn't find any.
Placebo mostly just means "good effect unrelated to intended treatment (active substance, surgical procedure etc)". That can include psychological effects of treatment (such as in pain management, depression, heart rate and blood pressure, and a handful of others). More commonly though, the placebo effect is just natural progression of the disease, errors in data collection (such as patients trying to be helpful and…
So I do agree these other effects exist, but I don't think they exist under the classification of placebo effect. It's possible one of these is at play in this study.