Learn English – I’m on doing something

In England, "How do you do?" was until recently a commonplace greeting. The correct response was, "How do you do?" This may be what you're thinking of.

Both "How are you?" and "How are you doing?" should generally be taken as a question, to which the reply is often, "Fine, thanks!" or, more formally, "Very well, thank you." However, the whole thing continues to confuse even English people, let alone visitors.

I would liken the use of the word Sound in this context to follow Oxford's definition 1.1 under the adj form.

Based on reason, sense, or judgment

Ex: the scientific content is sound

So considering that the idiom 'scapegoating' is generally used with a negative connotation the sentence is saying.

because "We cannot, like fear-ridden peasants of antiquity, hope to load all our crimes onto a goat and then drive the hapless animal into the desert." the use of idiom 'scapegoating' to show contempt for this type of behavior is reasonable/sensible or based on good judgement.