The UK would be in basic breach of WTO rules, in particular the MFN principle of equal treatment if it dropped tariffs at the Irish border but didn’t do the same for all other WTO members. This would open the door to recrimination and litigation precisely at the time the UK is trying to woo WTO members and use the organisation, however feebly, as a springboard for its own independent trade ambitions.
The other problem is that, without any obligation on the UK to enforce rules of origin and product regulations, all kinds of sharp practices and substandard goods (that are bad for jobs and consumers don’t want) could leak across the border and by extension the EU. Ireland would thus be liable for breaching the integrity of the single market by failing to control its border.
Let’s be clear: if the EU was compelled to establish a border (assuming the UK was relaxed about alienating WTO members), sure it might play well with the usual simpletons and their unrivaled victim complex who would proudly claim that “it was them, not us” but anyone with a half a brain cell would know why the situation arose and who was ultimately responsible for it.
As Alan Beattie says it’s a desperate and ignorant bluff - symptomatic of a tinpot, delboy approach to negotiations that continues to substitute for serious thinking and a real plan.