The telling thing in that interview I believe was Johnson's palpable irritation. It's quite clear he knew that he was on the wrong side of the argument, and he recognised the absurdity of the position he has been compelled to adopt due to the hubris and misjudgement of his puppet-master. That's why he was so irritated at the questions and any attempts to follow up, because he knows his bluster can only hold for so long. Any serious scrutiny and it will be exposed so he cannot permit it to happen.
I think the press conference will backfire, too. I fail to see how that will win over people to the side of Cummings, and that is what Johnson needed to do, because it's evident they are on the backfoot when it comes to the opinion of the majority of the public and the media. The arrogance and failure to engage with the details will enrage the journalists and ensure they pursue this until they get answers. I've never seen such a unanimity of opinion across the political spectrum, certainly not in recent years, as to the moral corruptness of the government's present position.