So if either one of those users can present an xpub that leads to only one of the 2 addresses, does that justify reasonable doubt from being the same person, and make it likely one of the private keys was manually imported?
This is an interesting question that I don't think we've ever tried to tackle here. My answer would be "no, it doesn't." All it concludes is that one private key was manually imported and it could still belong to the same person as the HD wallet. How often do people actually share their private keys with one another, anyway?
Now that we've given them a bunch of excuses to use I'm sure we'll find out there was a very solid reason to import the private keys or whatever. The fact that the user chose to completely ignore this (the more solid) part of the evidence is quite telling.
There is no chance to prove anything conclusively here but there is enough evidence and enough bullshit in this thread to make certain possibilities far more likely than others. If we're looking for absolute proof then we need to beg theymos for KYC and DNA tests.