is it a conspiracy theory that it was quoted by the lead investigator that Seth Rich was talking to Wikileaks on a computer that the hard drive was found to be missing on? Is it a conspiracy theory that the lead investigator was quoted as saying they were told to "stand down" on the investigation into Seth Rich's murder. These are facts. I don't care either way, but the facts surrounding Seth Rich, the people who refused to investigate it, it all seems very strange. I also would not think Julian Assange is making up a story about Seth Rich for his own deflection of events. I also don't think it is in his best interest to reveal his sources, albeit they might have been murdered because of it.
Yeah all of that is conspiracy theories. The "lead investigator" you refer to had his allegations taken down by Fox News because they "didn't represent the high standards of news" and he was told to stand down because of the threat of legal action by Rich's family.
I'd note that you fail to cite sources, but here's plenty explaining the conspiracy theory in depth.
Take a look at the facts rather than the interpretations of random events which are used to try and make it seem like he's involved in any way at all.
It's just a bunch of QAnon-lite conspiracy theories. It's a pity people fall for it so easily.