In my view the BA Twitter staff misunderstood EC261. The scenario they are suggesting would only begin to be true if BA had offered two solutions, one of which involved business class travel. The wording of the Regulation is totally clear, the relevant clause begins very simply: "If an operating air carrier places a passenger in a class lower than that for which the ticket was purchased...." then compensation is payable. They rebooked you, and placed you in a lower class, end of. If there wasn't a genuine dialogue then the only logical outcome is that BA "placed" the passenger. And note my emphasis on the word begin, even if BA had offered a CE option too far in the future I think further debate can be had.

Unlike delays, cancellations and re-routes there is no wriggle room. Furthermore the entire Regulation has a built in bias towards the passenger, it's not some balancing act between airline and passenger.