I don't quite understand the issue here. While it's hard to spot due to your camera position, he did curve his melee towards you.
It's important to understand that what you see isn't a correct representation of the truth. What you see is an image where your enemies are XX milliseconds in the past. And you are similarly far in the past on their screens.
What's happening looks essentially fine in the replay, and from their perspective it would have looked obviously correct. This doesn't really have anything to do with hitboxes, this is just lag compensation working as intended.
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