What you came across was BA's hidden luggage allowance for some status holders and premium class travellers. I say hidden because it used to be only those in the know who knew about the 32kg perk but now it's clearly there for all to see in the "Baggage Essential" page on ba.com.

Goodness knows why they didn't just put it there in the first place but as we all know IT is managed by strange people who do things in their own time. As for your e-ticket receipt...not sure you get one of them on a redemption.