Johnson would just keep parroting the line about playing politics anyway if Starmer did call for his resignation.When the only thing the leader of the opposition is demanding is an apology, how bad can the incident be?
At least Starmer dismissed the rubbish that it couldn't be prosecuted as it was "historical" and got Johnson to verbally commit to giving all the documentation/reports that come out of this "investigation" to the Met Police. Whether he'll actually do it is another thing.
One thing for sure is there would have been pictures taken at the parties, probably some with Johnson's face in and it will be a matter of time before they're "leaked", especially now Johnson has been repeating that he had no knowledge.