The author doesn’t counter a different argument. We never got an argument that babbling is nonlinguistic. So the author never countered such an argument.

There is no “generally accepted principle.” The fact babies babble isn’t a principle. Nor is the idea that babbling is nonlinguistic. So the author doesn’t undermine any principle in order to reach the conclusion that babbling is linguistic.

The author doesn’t describe a potential experimental test; he describes an actual test that was done and its results. The author also does not argue that we need a new test.

There are no assertions that an explanation is unlikely to be correct. Nobody argued against the idea that babbling is a linguistic task. So the author didn’t counter assertions that this explanation is incorrect.

The two possible interpretations are linguistic task or random sounds. The author presents studies suggesting babbling isn’t just random sounds (nonlinguistic vocalizations). This evidence supports the interpretation that babbling is linguistic.