Theresa May was a conventional politician whose government depended on the votes of the DUP, so she naturally refused to consider an EU offer on Brexit that screwed them over. Boris Johnson is anything but a conventional politician, and he has already lost his majority, so he made a deal which effectively puts the border between the EU and the UK in the Irish Sea. The DUP, of course, will not support this arrangement, which will have implications that go far beyond Brexit, but Boris doesn’t care; all he wants to do is leave the EU and win the coming election. Anything else is just collateral damage.
The government lost the vote on the Letwin amendment, but that doesn’t mean it won’t win in the end. Boris can probably count on every vote he got this time around, and some of the Letwin voters will almost certainly support his deal once they are certain that no-deal is completely off the table. Will it be enough? The final vote will be extremely close, and I make no predictions, except to say that the government probably has as much credibility now in Northern Ireland as Trump has in Syria, and that may matter even more than Brexit in the long run.