From May's letter... "As I explained when we met, the Political Declaration explicitly provides for the benefits of a customs union - no tariffs, fees, charges or quantitative restrictions across all sectors and no checks on rules of origin.
However, it also recognises the development of the UK's independent trade policy beyond our economic partnership with the EU blah blah"
I don't think she's conceding Corbyn's points on the customs union. It's possible... but I think more likely she is putting out more smoke and mirrors and billy bushlit.
The Political Declaration of course has no legal weight. And whilst it provides for those things, by my recollection it also provides for many other possibilities. That agreement is written like a horoscope... such that there is a little bit of something to satisfy everyone.
But when it comes down to final negotiation, there is no way that the EU will be allowing all the benefits of a customs union for all sectors AND the ability to strike unfettered trade deals with other countries - which is what she's trying to imply there. It will end up as either one or the other, not both... but by "providing for" them, it all supposed to look very appealing.
Once she's got Jeremy's vote, he'll be cast off the moment the departure from the EU is irrevocable.