My understanding is that they want parliamentary scrutiny of whatever Brexit deal is proposed rather than just rubber-stamping whatever the government manages to cobble together. The hope from Remoaners was that they may be able to mitigate some of the damage if the government's deal was a particularly bad one - or indeed no deal whatsoever. While they still have a verbal assurance that parliament will be consulted in the event of "no deal" (though I share Sam Goldwyn's views on the value of a verbal contract), in effect the long term future of the UK has been passed to a deeply divided Conservative Party propped up by the DUP (who, lest we forget is the Protestant Taliban).
Of course this cuts both ways because it would also give parliament the ability to reject a Brexit deal if, for example, it turned out to be a deal involving increased payments to the EU to remain in the EEA and customs union.
I'm still puzzled at the government's "Brexit at any cost" stance. They have committed the UK to a course of action, regardless of the consequences. That doesn't seem very sensible to me.