Essentially what that means is that Theresa May can ensure that Britain leaves in March if she is determined to do so, but that might well involve her accepting No Deal. So the key to understanding what is about to happen is inside May's head - would she actually do that?
In the mid-1990s, Alex Salmond and George Robertson took part in a televised head-to-head debate on the respective merits of independence and devolution. Towards the end of the programme, the audience member Lorraine Mann coined what became known as "the Lorraine Mann Question" - she asked each leader what their second preference would be for Scotland's constitutional status. In other words, she wanted to know whether Alex Salmond would prefer devolution or continued direct rule from London, and whether George Robertson would prefer direct rule from London or independence. Both men looked as if they wished the question had never been asked, but Mr Salmond answered it clearly - his second preference was devolution. Mr Robertson, by contrast, made a complete fool of himself - he refused to answer, and suggested (wrongly) that Lorraine Mann was an SNP plant.
Mr Robertson did, however, have the luxury of knowing that it was an academic question - he was never going to be forced to publicly reveal that he preferred London rule to independence. Theresa May isn't so lucky. She says the choice is between her deal, No Deal and No Brexit. If that's right, when and if her deal is defeated in the Commons, she's going to finally have to reveal what her own second preference is. We know what some of her Cabinet colleagues would do in her shoes - Andrea Leadsom's second preference would clearly be No Deal, and Amber Rudd's would be No Brexit. But which way will May jump? If she prefers No Deal, Brexit will probably happen on schedule, but if she prefers No Brexit, we could be looking at an extension of Article 50 and possibly a second referendum.