Dr.KarenChen said:

There is a difference between no evidence and evidence of no effect, and this subject is one where the two get swapped freely in both directions.

Coming at Dr.KarenChen’s question from a different direction. It is worth asking what the claim would look like if it were false. If nothing would look different, it is not a claim about the world and no amount of discussion will settle it.

Adding the numbers, since they settle part of this. One practical note: write down what you did and when, before you need it. Reconstructing a timeline from memory three months later is how people end up unable to answer the one question that would have resolved it.

Following on from Dr.KarenChen — and this may be the naive question:

How would you tell the difference between that and the alternative explanation?

Dr.PulmRoch said:

One practical note: write down what you did and when, before you need it.

Worth saying that the confident version of this is more useful to the person writing it than to the person reading it.

Closing the loop on my own question.

Update — I was wrong in the opening post, specifically about the direction. Correcting it here rather than editing it away.