It is as clear as mud, which is part of the problem. May had other red lines which were inconsistent with a no-friction Irish border.
However the following are clear:
- The UK’s original suggestion for the Irish border was a low-friction solution, which was rejected by the EU.
- The EU pushed strongly for the no-friction wording in the December agreement, and came up with the NI as part of EU customs union(*) with no option to unilaterally depart proposal in February. That is essentially preserved into this agreement, and has been the main sticking point in negotiations for 6 or so months.
(*: Which I assume means the central EU revenues would get 80% of all tariff income on tariffs for imports into NI, including from the UK. If anyone has any evidence otherwise I would love to see it. This is an issue I haven’t really seen discussed, and appears to be the most obviously odious aspect of the EU proposals.)
It’s entirely unclear to me whether May has folded to the EU’s imperialist demands or May has used them to lever a soft brexit deal which nevertheless ends freedom of movement into being. Which is certainly an achievement, but not, I think, the achievement her voters were expecting.
I note that one way out of the backstop for the UK is Irish unification. Which would make the problem of keeping the peace on the island of Ireland entirely the EU’s. And this withdrawal agreement might make that an actual challenge - if anything will goad the most extreme unionists back into terror it would be this kind of process. As I said before, short termism on all sides.
EDIT: The first two thirds of this is a fair summary of my position in a little more detail: https://www.conservativehome.com/thetorydiary/2018/11/tactical-wins-strategic-defeat-mays-deal-binds-us-to-the-backstop-and-threatens-the-future-of-our-precious-union.html
(The last third is a typical brexiteer advancing of alternatives to which the EU has no reason to agree to, and has said that it will not agree to, as realistic alternatives)