Are the Hardy-Littlewood “rules” unethical?
I've always interpreted such remarks as meaning that the individual papers were just progress reports on their larger enterprise, to which both made irreplaceable contributions, and that they had no interest in keeping track of moment-by-moment relative contributions.

In particular, "on average", there was no misattribution or false credit.

(Further, the "not contributed the least" may easily be hyperbole, just to make the point.)

For that matter, if one is in regular correspondence with another, how to attribute ideas that develop gradually? I think their solution was entirely reasonable.