The EU have always 'offered' a UK wide customs union as potential backstop as long as it meets their concerns.
Back on the 26th of July, Barnier said this at a press conference.
Not really sure why it's been dressed as some concession. It's been on the table since the start. Hopefully we can now get to the next moment of reality which is whatever is set as the backstop is almost certainly going to be the fundamental base of the future long term relationship. There's no FTA which will remove the Irish border issues.