Jeremy Corbyn has unveiled a leaked Treasury document showing that, despite Boris Johnson’s assurances, there will be customs checks on goods going between Northern Ireland and the rest of the United Kingdom and vice versa.

His argument is that, as Johnson has already broken his word by putting a customs and regulatory border in the Irish Sea, we can’t possibly trust him to keep his word on Brexit. Corbyn’s case is unimpeachable, copper-bottomed, seaworthy: whatever synonym for “unarguable” you want, it fits.

But his actual prop to make that case tells us nothing new at all – it is the first official document saying that as well as checks going east to west (that is, between Northern Ireland and the rest of the United Kingdom) there will be checks going west to east (that is, between the United Kingdom and Northern Ireland). But that is “news” only in the sense that someone telling you “you have to take the stairs to get to the top of the tower” might tell you that you have to take the stairs to get back down again – it follows inexorably from the first point.

It’s similar to Corbyn’s first big secret document, which only told us what anyone who had looked at the detail of US-UK trade knew: that the only way to have a meaningful US-UK trade deal is to put British agriculture and pharmaceuticals on the table.

For Labour, the two documents are inextricably linked. Labour’s case is that, because Johnson has been shown he can’t be trusted on the customs and regulatory border in the Irish Sea, he also can’t be trusted on keeping the NHS out of a US-UK trade deal, or increasing the number of police on the beat, or any other promise you care to name. And again, the policy case here is unarguable: there are, in my view, good political reasons to think that in practice Boris Johnson will never be able to overcome public resistance to breaking these promises. But the pattern across these documents is clear: that policy assurances from Boris Johnson tend not to mean very much.

While both documents perfectly illustrate the case Labour wants to make, neither document is “new”. But Team Corbyn know full well that the only way to get most of the press, particularly our all-important general interest broadcasters, to cover policy is to add the words “Top Secret” in big shiny letters on it. So after an NHS story that revealed nothing we already know, we have a customs and regulatory story that reveals nothing that isn’t in Boris Johnson’s withdrawal agreement. Yet without these leaks, coverage of the content of Johnson’s Brexit deal would be even thinner than it currently is.

One of the troubling things about this election is that, outside of the specialist press and Sky News, the policy detail of Johnson’s Brexit plan has barely been scrutinised. That matters not just because of the election and how it might change the result but what happens next. Trade policy isn’t, largely, about things that are hidden from view: it’s about the granular detail of things like the withdrawal agreement – the kind of thing that the BBC’s Brexitcast podcast, the most influential Brexit podcast, frequently describes as “nerdy”.

It’s not nerdy – it’s essential, and it shouldn’t require the leader of the opposition to treat things we already know as top secret for it to be covered properly.