All nations have uncomfortable aspects of their history that they need to come to terms with. It was a dumb thing to say for a President, especially if there's no proof, but it's also a dumb thing to get upset over for a Prime Minister when it is a part of their history.
I am sure that Biden was being super spiteful and melicious about it and was deliberately lying rather than passing down hearsay from his family he overheard throughout his life /s.
No, wasn't a good look politically, and I do agree with New Guinea the US should do more to recover our fallen soldiers whose remains are still missing on foreign soil. Glad this conversation can put a spotlight there. But this is a minor gaff, simple as that.