He’s not kidding. He has done Laertes wrong. Like – there’s not even a question. He has unquestionably behaved badly.
Sometimes characters do things that could be justified from some other perspective – like, sure, I broke your sister’s heart but it was for the greater good or whatever.
In this case, he behaved like an ass at Ophelia’s grave and that would SEEM to be what he’s apologizing for – being a self-important narcissistic selfish ass – but he has ALSO killed this guy’s dad in a kind of ignoble manner – which is probably the worse sin. They both know that. And maybe they are pretending to talk about the grave leaping when they are, in fact, talking about the murdering.