But they are refuting it, just more weakly. In any case, not to refute it would be gross political malpractice and I'm sure Biden has better advisors than that.
Political communications like that must be parsed extremely precisely if you don't want to be tricked. The phrase "as alleged" is a huge loophole. The thread that the sibling comment linked to goes into this: https://twitter.com/aaronjmate/status/1316442180358213632
Of course you're right in a logical or philosophical sense, but in a political sense it certainly does. If the emails were fake, it would be politically insane not to say so. The Biden campaign can easily find this out from Hunter, so the fact that they haven't said it means either that (1) the emails are authentic (though they may still have been altered), or (2) something else is going on that we don't know about and is unusual.
It seems that the Biden campaign isn't denying that the emails are authentic. That would be strong evidence that they are, assuming this hasn't changed in the last 24 hours: https://twitter.com/lhfang/status/1316461339842289664
To be clear, this is a separate question from how the emails were obtained. That story is beyond weird and surely bogus.
To be fair, it also seems to me an indication that Biden is more honest than Trump. Trump would just say the emails were fake whether they were or not.