@GavinBarwell isn’t quite right: the requirement for checks on GB to NI goods isn’t implied. It’s express: but it is carefully concealed so that those not paying close attention and without a set of EU legislation to hand won’t see it.
The key is this dry sounding reference in Article 5(3) of the NI protocol, which we are told sets out legislation that will apply in NI. 
The drafter carefully doesn’t tell you (or a careless journalist) what Regulation 952/2013 is. It’s the Union Customs Code: the huge Regulation prescribing the full panoply of EU customs law. And Art 5(2) of that Regulation includes the UCC and all its satellite legislation.
That includes all the rules on checks and enforcement.
That is just one example of the deliberately deceptive drafting of Article 5 of the Protocol. See also Art 5(1).
That looks like the general rule: no tariffs on GB to NI goods. But it isn’t. It’s the exception.
That’s because any GB export to NI is defined to be “at risk of subsequently moved into the EU” unless and until it is “established” (ie proved, presumably with paperwork) that it isn’t. See Art 5(2).
So what is carefully drafted to look like the exception is in fact the rule.
I leave it to you to guess why the drafters felt the need to draft in such a way as to hide from journalists and politicians giving the text a casual read what the agreement actually, on analysis, clearly says.
FAO @SophyRidgeSky @RidgeOnSunday, for the next time she is given this response.
