The null hypothesis is that we don't know why/how they contracted the virus. Parent doesn't have to know it's true, or be an expert in anything.

Their exact statement was: "We don't really have enough contact tracing data to reliably conclude"

Your argument, then, is that one can make a confident affirmative argument about the null hypothesis absent any evidence and be correct in so doing...?

Humans have been dealing with respiratory diseases since well before the dawn of recorded history. An article on nytimes.com proves nothing, but the null hypothesis (for a trained and educated medical professional) is not going to be "I dunno". The null hypothesis is "this will behave like [most similar virus I know about] and humans action will be causing spread in a manner following the epidemiological studies I have read".

COVID shouldn't be so much of a mystery any more, it has been 12 months of the most focused study the world medical profession has ever done. The only thing holding our knowledge back of the short term behaviour of COVID is ourselves at this point. We've had time to watch the disease play out multiple times.