Pretty weak from what I can tell. When I first read the accusations I expected it to be some blatant thing, but it basically boils down to claims he lifted the idea of Moira living multiple lives from the book of novelist Claire North, who he's praised in the past. But...that's hardly a unique trope. It's been done time and again. For it to be "plagiarism" in any real sense, it would need to be a unique idea to her work to begin with and, well, it isn't. It's been around for decades. Whose to say she didn't rip the idea off from All You Need is Kill? Or Groundhog's Day? Ken Grimwood was using that plot device in Replay over three decades ago. Claire North didn't invent that plot device so she can't really lay claim to any kind of ownership of it. Hell, author Jonathan Lupoff tried to sue Columbia Pictures when Groundhog's Day came out because he felt it ripped off a story of his from the early 1970's, but was basically told by lawyers there wasn't a case and dropped it.
For that reason, plot devices are rarely actionable as plagiarism in legal cases unless the plot that contains them is beat for beat similar to something pre-existing. A general concept (reliving one's live over and over again) isn't unique enough to be "stolen" per se. For Hickman to have plagiarized North, it would have to be way more specific to her work than that (specific plot beats, characters, etc), and I don't see where it is. North's story is about two guys having a multi-lifetime rivalry and involves a secret society of people who reincarnate. There's nothing like that in Hickman's work, really. They're both using the same idea but it seems to begin and end there. And many others before them have used a similar idea. So...much ado about nothing in the end, I think.