It looks like some posters are just writing utter nonsense.
If we do not have control over the border between NI and RoI, (assuming we are not in a/the customs union) what is the point of a border control anywhere else, Calais, Dover, etc? It is not just about people movement. RoI is not in the Schengen area so movement of people into RoI is controlled at RoI borders already. But EU citizens have relatively free movement.
I think the UK are intending on prevaricating until Brexit happens, without proposing or implementing a hard border, which will kind of force the RoI/EU to implement the hard border. The UK government will the point the finger at the EU and blame them for the reintroduction of a hard border.
Except, of course, the EU border will only control movement of goods or people into the RoI/EU, not out of. The UK government will be obliged to implement their own controls for any inward movement of goods or people.
It will result in chaos! Of that there can be no doubt.