The Worldwide Disruption Agreement is "intended for enactment following events that cause severe disruption to the worldwide operation" as quoted from the document itself. It enables flexibility of cabin crew rosters within defined limits so that BA can more effectively deal with said 'event' than would otherwise be the case.
IMO the current situation clearly constitutes 'severe disruption' and therefore the agreement should be implemented.
However, I believe BASSA are laying the blame squarely with BA themselves and as such seem to be saying that the situation is 'self-inflicted' and therefore will not sanction its implementation.
In any event BASSA have the ability to give an 'alleviation' to laid down agreements if they so chose.
The words cutting your nose off to spite your face spring to mind.