Indeed but you cant separate people from a border, for example there are Irish citizens as opposed to Uk citizens across the border. Thats enshrined in ROI constitution and GFA, they are essentially EU citizens.
Now the Uk dont have to put up a border technically you are correct under WTO rules, they do have to secure the border, the specifics of that are broad and loose. However that would have the knock on effect of Belfast being treated differently to say Liverpool or Dover etc. So different regularity or customs rules throughout its claimed border.
Additionally the Uk would be breach of WTO’s non-discrimination rules, particularly “most-favored-nation” treatment (MFN), which means treating one’s trading partners equally. Suppose the UK and EU trade on WTO terms after Brexit. Suppose American fidget spinner arrive in the UK at an English port have to go through controls, but Irish fidget spinners crossing the border into Northern Ireland (also the UK) do not. Then the US could complain that Irish fidget spinners were discriminated against. They weren’t given equal treatment with Irish fidget spinners when they entered the UK.
The US might seek a legal ruling in WTO dispute settlement. Months or years later, the ruling might conclude that the UK had discriminated. So either checks at the English ports would have to be dropped, or checks at the Irish border would have to be set up.
In other words, while no WTO rule actually says the UK will have to set up border checks, the non-discrimination rule may force it to.
