Thinking you wouldn't be badly affected (and by Johnson's demographic, statistically it was likely he wouldn't have been), doesn't mean you think it doesn't affect others badly.

I'm in my twenties and fit, and going by the vast majority of cases that means I'll have very mild symptoms. That doesn't prevent me being considerate how badly it affects others.

You're also completely ignoring the context that at that point the country had been locked down: you couldn't leave your house except to get supplies and 1 hour of exercise. Does that sound like the decision of a man that denied the gravity of Covid?