Dominic Cummings likes his metaphors of chaos and destruction. He used to take the pin out of an imaginary grenade before throwing it over his shoulder into a meeting room he had just left. Today, an ally is quoted as saying that he is going to try to “napalm” the prime minister.

Cummings also enjoys the drama of politics and seems to be encouraging the natural tendency of journalists to build up his appearance before the health and science joint select committee on Wednesday as the moment he will detonate the bomb he has placed under the government.

The advance publicity, much of it generated by himself on his blog and on Twitter, promises much. There is the outline of the “I was right but Boris wouldn’t listen” case on handling the coronavirus, and even a “smoking gun” document, the only copy of which is in Cummings’s possession, that he will give to the committee at 9.30 on Wednesday morning.

But I wonder if these are all the ingredients of a box-set series finale that will disappoint the viewers. It will certainly make Prime Minister’s Questions interesting, giving Keir Starmer only a couple of hours to prepare, but if Cummings really had the means to destroy Boris Johnson I think he would have deployed them by now.

What we are likely to get on Wednesday is an important preview of the public inquiry, going over ground that has been well covered, with an important witness who was in the room where it happened, including sitting in on some of those early meetings of the Scientific Advisory Group on Emergencies (Sage).

Cummings’s account has the advantage for him that it fits with the dominant media story, which is that Johnson was too slow to lock down, first in March and again in the autumn of last year. But the problem for him is that, because he was there at the time, Cummings knows that even if it is true — which is not certain — it does not mean that Johnson was at fault.

Cummings knows that the prime minister was advised by Sage to delay restrictions in March, and that there were good arguments in September against a “circuit breaker”. His tweets suggest that he thought the behavioural scientists (“charlatans”) got it wrong in suggesting that we couldn’t do what Taiwan did — close the borders; rigorous test, trace and isolate — but he knows that “it won’t work here” was the advice that Sir Patrick Vallance and Professor Chris Whitty gave Johnson. Yet at the same time he praises Sir Patrick for taking the vaccines programme out of the Department of Health, so he knows that part of the story is not even as simple as “the scientific advisers got it wrong”.

He knows that deriding “our joke borders policy” puts him on the right side of public opinion. “Close the borders” is the one idea that has united the nation at all times during the crisis, regardless of whether it was possible or whether it would work. By February last year it was already much too late to stop the virus spreading in the UK, and there is less point to it now.

However, telling people what they think “everyone knows” is a good way of overcoming the Barnard Castle credibility gap. Public opinion would otherwise take a sceptical view of someone who seemed to interpret lockdown rules to suit himself. Indeed, few people would take him seriously at all if it weren’t that so many journalists conspire in the myth that he won the 2019 election and got Brexit done — he did not: he nearly blew up the whole thing, while Isaac Levido had total control of the election campaign.

Having been there, Cummings knows well how chaotic and difficult decision-making is, and how different from the simplifications imposed retrospectively. In his tweets, he points out that the legal lockdown on 23 March was not the critical point: the dramatic changes in behaviour in the previous two weeks were what mattered. He thinks they should have come sooner, but it is hard to see how they could have happened more than a few days earlier than they did.

Perhaps “the only copy of a crucial historical document from Covid decision-making” that Cummings claims to have will change everything. But it seems unlikely. It would be odd for there to be only one copy of an important document. Unless it records that Johnson said “Let the bodies pile high” as a considered policy decision rather than a momentary expression of frustration, it may not add much to the ocean of evidence that the public inquiry will eventually wade through.