Freedom of speech is a legal concept that clearly doesn't cover CSAM. Free speech is a principle but it also doesn't cover CSAM. Fire in a crowded theater doesn't actually work as a legal defense but obscenity does
That's another reason I prefer the term "free exchange of ideas"; by using different wording it helps avoid the confusion created by people conflating the general principal of such freedoms with any specific legal provisions that exist in the U.S. constitution. (Though I agree in this case my wording is in agreement with how the courts define "freedom of speech" in practice.)
As discussed ITA, that case was later overturned; and it’s worth remembering that its origin was as an analogy to justify criminalizing pamphleteering against the draft [0] (one could not imagine a more salient example of “political speech”)