If May's offer is anything like that reported in the media then there is no point in continuing negotiations. If accepted it clearly isn't leaving, since we'd still be conforming to EU demands, and the public will have been betrayed by a government that will have already shown disdain for democracy by putting the offer together in the first place. If rejected there can surely be no further capitulation.
If the hope was that the EU will reject and we then choose to just leave, as would be the only remaining honourable option, then the EU could change their mind and offer to accept the abominable suggestion the UK government themselves put forward, and which was initially rejected, after all; and any government bluff would then have been called. It'd have to either agree to their own suggestion, or reject their own suggestion, claiming it's too late and off the table.
No idea what the leaver MPs agreed on this for. But the issue won't be sorted down this path. Negotiations seem to have failed so maybe the best thing now is to simply get on with the 'out' part and sort out the 'better than WTO' trade agreements afterwards.