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.
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.