Number10 is the home to the Prime Minister. Where was Johnson on the night when the party or for Johnson an 'event' occurred? He must have been out for the whole night, with his wife and child, while his staff in Downing Street had food, drinks and played party games. Otherwise, he could vouch that 'rules were not broken' in his residence and not have to rely on 'staff told me no rules were broken'.
Interesting that the MET said 'no evidence'. Didn't the police at the gates see large quantities of food and drinks arriving and not be suspicious what was going on? Seems like the MET are covering their backs and trying to dodge criticism for their inaction in not stopping a party right under their noses.