The PM and Dominic Cummings had two obstacles:
1) The ERG
2) Majority against No Deal.
So they did two things.
1) Staked future of Tory party on need to leave by 31st
2) Put ERG on notice by making an example of moderate Tories
They will vote for the deal and be allowed back in, all is forgiven etc.
The ERG are caught. They committed to leaving by 31st. And they would be fired if they vote against BJ's deal.
Which means Boris Johnson might get his deal passed, if the DUP and all of the ERG fold.