When Trudeau was asked about this the first time, he said that in his memory, nothing negative had happened that day. Then he said something did happen, but he's not sure what, and he apologized anyway.
The reporter he groped released a statement last week confirming that it did happen, that he apologized, and that was the end of it.
The real issue here is that Trudeau has been very heavy-handed in dealing with alleged past indiscretions by members of his own party, and by the standards he has publicly set, he should resign. He won't resign, but this has made him look terribly bad, and it's been very satisfying to watch.