Everything he says pretty much sounds serious and I can't differentiate when he is joking and when he is serious.
My INFJ father jokes this way. He's also a lawyer, which could be part of the reason people take him so seriously. He's always quiet, and when he does end up talking it's about politics or something serious. But he'll joke around, and people get confused when he is joking.
He'll say things to young people my age, my friends and shit, in a serious tone of voice
"Well, there was a dwarf at the door asking for you when you were away." He says it so seriously that one of my friends, ENFP, responded,
"What?! You mean, like, a midget? There was a midget looking for me?" And Wesley sounded all panicked. My dad has a straight face. Seriously, no emotion on it.
"Yes, he was looking for you. Said you were his only hope, something about looking for a cure,"
Wesley's eyes were big, he was all freaked out.
"Oh my god?! I don't know any short people. How does he know who I am?"
I had to step in and tell him that my Dad was joking, and Wesley was embarassed that he had been fooled.
It sounds silly at that level, but my father has devised all sorts of lies that I often excerise. He convinced a lot of his friends, adults in their 40's and 50's, that the holy trinity was going to be split into the holy quad to attracted younger people to the catholic church, they were going to add Colonel Sanders, from KFC, as the third one. Then you could drive up to KFC, have confessions to father Colonel Sanders, and pick up dinner as you go. Everyone believed him. They were skeptical 'Oh, you're just joking' to him and then he pulled out a fake article that looked convincing and had everyone fooled.
That kinda sounds like the type of joking you're talking about.