The real point, I think, is that the position on GB to NI trade (the left hand side of the HMT table here) has been known for some time. Most NI imports from GB (and from the rest of the world) will be subject to the full panoply of EU customs rules and tariffs. 
That is hard to reconcile with the Government’s claim that NI is not to be in the EU customs territory (a claim it has to make to avoid the prohibition in s55 of the Taxation (Cross-border Trade) Act 2018). legislation.gov.uk/ukpga/2018/22/…
See here for legal analysis and facts.
So looked at from Cairnryan or Holyhead, there is a great big barrier between you and your ability to sell goods in NI.
What the table vividly shows is that there are likely to be substantial barriers to West-East trade, despite Johnson’s claims that there wouldn’t be. Looked at from Larne, there is a barrier down the Irish Sea - a barrier you don’t face if you sell across the Celtic Sea to 🇫🇷.
But it’s a great thread, and another nail in the coffin of #GetBrexitDone as a credible slogan.
