I think some don't seem to agree on what an open border, vs a hard border, means; or they wouldn't insist no-deal means a hard border. Quote from the PM herself, "the measures intended to ensure that whatever else happens, there will be no return to a visible border between Northern Ireland and the Irish Republic after the UK leaves the EU". So an open border is simply allowing traffic/pedestrians to pass through as if no border was there. It has nothing to do with having a system in place to ensure correct taxes, tariffs, standards are applied. The systems for achieving this already exist, as folk have indicated above.