I think you may have got it right first time.

With the UK being tied to WTO rules after a hard Brexit, the only way to avoid a hard border in Ireland would be for Northern Ireland to become a member of the WTO in its own right, separate from the rest of the UK. I somehow can't see the DUP going for that, though it would make some sense. Or rather, because it would make some sense, as a step towards a two Ireland constitutional arrangement outside the UK. Here's an RTÉ story on that.