My understanding is that the purported emails were sent to Biden. He can't really do anything to control what is sent to him, so verifying that these emails were sent doesn't really prove anything. If he had forwarded the emails in a way that indicated his agreement, or if they had been sent by someone else connected to the candidate, that would be a situation in which DKIM would be relevant.
> He can't really do anything to control what is sent to him
In which case the whole situation is nonsense and moot.
But it appears that some of them are from Biden - "The New York Post cited a purported email from Hunter Biden in August 2017 indicating he was receiving a $10m annual fee from a Chinese billionaire" - in which case the DKIM headers would be a simple check on authenticity, no?
Sure, if he is alleged to have sent an incriminating email, the headers should be published along with the exact contents so that the public may judge for ourselves. This was done for the Clinton 2016 emails that clearly showed undemocratic interference in the primary elections. So, the fact that they haven't been published in this case does make the whole story less believable. The journalists involved this time probably aren't as smart as Wikileaks, but that's no reason their reporting shouldn't be held to the same standard.
My understanding is that the purported emails were sent to Biden. He can't really do anything to control what is sent to him, so verifying that these emails were sent doesn't really prove anything. If he had forwarded the emails in a way that indicated his agreement, or if they had been sent by someone else connected to the candidate, that would be a situation in which DKIM would be relevant.