But I think it is pretty obvious that pretty much all the relevant authors of the article must know that without any fabrication of the data, they don't get any hockey stick from the trees. It means that it can't be an innocent mistake and all of them, and not just Keith Briffa, are fraudsters who know very well what they're doing and why they're doing it.
That is, Motl alleges that Briffa et al are not simply wrong (which they're not), but that they have intentionally made up data. That is, instead of using numbers they (or someone) measured, they sat down and wrote out numbers that were guaranteed to give them the result they "wanted." Of course, Motl has no proof whatsoever, and doesn't even attempt to offer any.
If that's not defamatory I don't know what is. I'm waiting for the day when a scientist sues someone like Motl for a post like this. I don't think it would be an entirely bad idea.