They can't, which is the point.At the moment Norway pays dues as if she was a full member, allows free movement of labor, accepts the rulings of the european court and has an arrangement which is, in all but name, customs union and free martket.
How can Barnier give more generous terms to the UK?
The Norwegians, who have been very quiet up to now, would be screaming blue murder!
Only one of 2 things can happen, either the UK refuses customs union, free market, free movement etc, in which case today's "agreement" of no border between RoI & NI and no border between NI & the rest of the UK can't happen or the UK does agree with the above, in which case we'd have got everything we wanted anyway without today's "agreement".
In other words today's "agreement" is completely meaningless either way.