[snrplfth]

I can almost guarantee you that Scott Alexander is aware of who the Jacobites and the Jacobins were and what the difference is. The blog is constantly bringing up all manner of historical and political esoterica. He's making a joke based on the name resemblance.

[NumberSix]

It doesn't read like a joke. It reads like a mistake or perhaps he is assuming his audience won't be aware of who the Jacobites were and will find it funny.

[snrplfth]

I've read this blog for years. This is exactly the kind of joke he makes all the time; in describing or referring to something, he folds in a fact which, while not literally true, is a funny way of imagining it to be, and which is expected to be obvious to the readership. It's kind of like the practice of Using Capital Letters to refer to a Self-Serious Idea of Something.

It's just the way he writes.

[DoreenMichele]

I have never read his blog. The fact that he is playing with language and in no way confused seems clear as day to me.