I'd like to hear from Johnson himself on this, following his recovery. It may be that he was badly-advised, or that he simply failed to appreciate the scale of the coming threat, or some other oversight, and so felt it unnecessary for him to chair or participate in the relevant Cobra meetings. A clear mistake, even at the time, but I'd still like to understand his position on it.
I'm reminded a little of the time when Eden, Foreign Secretary in 1936, approached Baldwin about the growing threat from Germany, and was told that "I would rather you didn't bother me about foreign matters just now", as Baldwin was more concerned about the Simpson/Edward VIII affair... sometimes a PM's priorities are misdirected.