IMO, these thoughts are exactly why he is doing it. It gets people thinking like that. He wouldn't do it if this was the case, right?
I just think that he does not care about it really. He knows the DNA is not of any intruders. He knows that some foreign DNA was found. Foreign DNA could be found in anyone's house. He's probably just thinking that let people have their fun with it. Let them discuss and think their theories, let them test it and write articles. Let's keep it interesting. He already knows that the DNA is insignificant and is not connected to the case in any way. It got there probably from the factory where it was manufactured, packaging, transport. Or maybe someone took it out of the package and touched (or sneezed close to it) at the store when looking at it. It at least should be considered a possibility, right?
I don't think that's a possibility any longer, not since the Bode report. Factory worker should not be considered a realistic scenario, but that's irrelevant to the main point I'm making here. Let's just leave it at, if John or another Ramsey is the killer, then John knows the DNA didn't come from the killer, and have that as our postulation.
My point is, the random person who is the owner of this DNA probably is not a criminal but just a simple person like all of us, and the chance to match the DNA to anyone is very small. Would a random, innocent person who has not committed any criminal acts in his past and has no record, would have his DNA at any database?
But that is already the status quo. UM1 has been in CODIS since 2003, and obviously nobody has matched against it for 20 years. What John is requesting - genetic genealogy - is a tool to find a suspect who isn't in any database, but can be found through his relatives, who have uploaded their genetic information to a database. The Golden State Killer wasn't in a database, but was still found with this method.
And even if there is a match and the person has an alibi - they would only have to say that the person who matched the DNA (or some part of the DNA) had a solid alibi and is not connected to the crime. What would it change for John? If he already knew that anyway.
The point isn't what John knows, it's what the public knows. If the public knows UM1 wasn't the killer, John has lost his best argument for the innocence of his family.
He'll just say that this is unfortunate and not what they hoped for. Or, that the DNA match is false or inconclusive. Or, that they will do more tests.
Which will not work. UM1 is a strong piece of evidence - I would say the strongest piece of evidence in the case - and it's its existence that made Lacy exonerate them as well as Beckner state that UM1 is their number one suspect until proven otherwise. Whatever you think of those statements, they have had a big effect on the public at large. If all John cared about was public perception, he could have left it there - he would know this is as good as it gets for them. Pushing for an identification of a person that he knows wasn't in the house at the time of the murder is extremely counterproductive.
So the only reason for him to keep talking about the DNA and testing it to find a match, IMO, is that it makes the public see him as a good father who will forever search for the kidnapper/killer of his daughter.
Or he could just have left well enough alone, say he has accepted they won't find the killer, share his grief at anniversaries, criticize the police for botching the case. After the CBS fiasco it's not like there was a major push for a documentary accusing the Ramseys anyway, so what reason would he have for this latest campaign - unless he actually is telling the truth and recent progress in DNA technology - with famous solved cases like the Golden State Killer - has given him hope that the actual killer can be found?