To my regret, I didn't pursue it. I bit the bullet and bought the seats (was only going to pay for a berth on the return leg) for fear of them going up whilst complaining...(they did just days later).
Just had a look and at least the 2T website has been changed. Now says
"Two Together Railcard discounted tickets are accepted on Sleeper services, providing you board the train before 0430. Discounts are not available on Sleeper berths or travel/berth inclusive tickets."
Then confusingly
Caledonian Sleeper Tickets
- 1/3 off Caledonian Sleeper services when boarding before 4:30
Um, can't you board after 4:30 if it's the weekend or the evening for local travel? Although of course I see from this board they're not keen on that sort of thing.