Parent is arguing that they need to witness language use between members of their own species, and then not learn, in order to show they are incapable of language use.

The first human language user managed without this. (Though, if used for communication, it takes two... so there couldn't have been a first. OTOH there's an argument that language is primarily for thought, not communication, so it only takes one person. Secondly, evolution doesn't usually build a complete perfect system, but iterates, gradually improving. A puzzle here is that abstraction seems to be all or nothing, you can't have part abstraction. One resolution is that the necessary neuro-machinary was built up for other purposes, and abstraction emerged when some crucial capstone happened to be put into place. Or... maybe you can have some kind of "partial" abstraction...?)