"The EU never has needed to implement a hard border but threatens to do so for two reasons."
The EU has no authority to implement and police the border on the island of Ireland. Since neither the UK nor Ireland has any intention whatsoever of doing so, the question that needs asking (and one MP did so in Parliament three or four weeks ago but received no reply) is "Who exactly is going to implement and police the border?"
The border question has simply been weaponised by the EU in an attempt to force us to remain in a customs union with them. Among their biggest fears of Brexit (and, despite assertions to the contrary, they have quite a few) is that we become globally competitive in a way that we never can whilst we are members and that we undercut the members of their protectionist racket. Unfortunately our spineless government has acquiesced to their every demand. So eager are they for the UK to remain as "quasi-members" of the UK that their powers of reasoning seem to have flown out of the window.