I think what they're saying is that Britain signed a treaty (the Good Friday agreement) that guarantees an open border on the island of Ireland, and an Article 50 withdrawal from the EU doesn't change that. Since the EU doesn't want Britain to leave, it's Britain's problem to find a way to maintain its treaty obligations to the people of the island of Ireland. (If I understand this correctly, the EU can't legally waive that even if they wanted to, a point which will come in handy if the British government asks them to.)

This feels like a "pick at most two out of three," where the three are the Good Friday agreement, the continued customs union between northern Ireland and the rest of the UK, and Brexit. The EU has the advantage here, because they only want the first of those things; it's Westminster that thinks it can have fast, cheap, and good.

(You're likely right that they don't want an open border, but they did sign that treaty.)