He apparently admitted it in texts to his girlfriend. They would have some value in court but a jury would hear it in a context of other evidence that would put the causal link between the alleged punch and the girl's death in doubt.
He went .'no comment' at police interview and may well have, as is his right, refused to take the stand at trial.
I understand the analogy with pub brawls where somebody bangs their head on a kerb or whatever and they're charged with manslaughter. Causal link there is crystal clear.
It's been said before that the difference between manslaughter and man's laughter is merely a bit of punctuation.
Hard cases, either way, are why we have a system to decide what get's prosecuted.