As usual, you make creative comments.
I have also heard this from a reliable source internally.
Several months ago, starting at some point last year, it started to become impossible for me to use United miles on the United website to book mileage ticket space on SAS flights. It has not worked ever since for me, and I am usually looking several times a week. During this entire block I have been buying Lifemiles to redeem for mileage ticket space on SAS flights because using UA miles has not been working even as A3, AC, AV, TK and other programs had no problem booking the SAS flights with mileage ticket space.
This blocking took place months before SAS started substituting away from UA and into DL. Using SAS miles for UA flights was working ok into at least last month.