I think you're right to an extent - I don't think most parents speak multiple languages to the baby. Only when the baby grows into a child do they start learning multiple languages.
I'm inclined to think most parents from multilingual households do speak multiple languages with one another and with other adults; hence the baby is exposed to multiple languages.
(My interpretation of the article is that it pertains to languages heard by the baby, not per se the language(s) the parents choose to use when speaking directly to the baby).
Also there is research that suggests that the sounds made by babies already copy the unique speech patterns of the parent's native tongue(s) at a very early age.
In my own experience, when I could start to distinguish individual words spoken by both my children, these words were in fact from both my native language and English; to me it does not instinctively feel as if there is a demarcation from a baby growing into a child where there is suddenly a point at which they start absorbing multiple languages -- babies seem to absorb everything.
In fact, thinking back to my own childhood growing up in South Africa, I never made the distinction in my mind about there being words from multiple languages, until a much later age (4 or 5 probably); I guess before that it felt simply more like a "continuum" of language, without it mattering that some words had different phonics/rules associated with them.
