A narrower follow-up, since the general answer is now clear:

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

TirzTom said:

For anyone reading later: the numbers in this thread are worth checking against a primary source before you act on them, including mine.

This is where I part company with the consensus forming above. Not convinced. The evidence being cited is consistent with the conclusion and also consistent with two other conclusions nobody has ruled out.

Correct me if the detail matters more than I have assumed.

TirzTom said:

For anyone reading later: the numbers in this thread are worth checking against a primary source before you act on them, including mine.

Agreed, and the size of the effect matters as much as its existence. Something real and small gets treated here as though it were real and decisive.

TirzTom said:

For anyone reading later: the numbers in this thread are worth checking against a primary source before you act on them, including mine.

This answered a question I did not know how to ask. Adding it to my notes with a link back to this thread.

Closing the loop on my own question.

Follow-up — I put something in the gap where food used to be rather than waiting for the gap to close on its own. That was the part nobody told me.