Then why did the experts not realise that before?
I have no reason to believe they didn't.
however, I do believe that there was a shortage of PPE especially masks, and government directives were intended to make hospital and care workers make do with what they could get, without kicking up a fuss; and to quieten the general public so that they did not demand protection that was unavailable and riot in the streets or refuse to go to work.
You may recall some government politician accusing hospital workers of using equipment extravagantly.
It's possible that after Cummings, and his subordinate Johnson, have been kicked out, the nations top nurses and doctors will say he behaved recklessly and in contravention of government rules. This will not be because they have changed their minds. It will be because they are no longer gagged.