"They get sick first. They're done. Now we are only left with the more resistant people."

That, combined with a downward slope for the population-wide "sterilizing" immunity that has dropped all summer long could account for the result (otherwise there should be a rise between April in June, followed by a steeper drop in July and August, if natural increase in infectious potency was working alone).

In the end the data can't possibly support Delta being more or less infectious than Alpha because Delta and Alpha aren't working with the same clay.