I guess Ne types find it easier to catch up to the sarcasm but doesn't mean Sensing types don't. Probably Si's would completely miss the whole joke. Or we just have a humour that other types find hard to catch on.
I once made a joke to an ISFJ, saying "Oh wow, isn't that an accurate representation of yourself!" (we were making self-sculptures for art and she's given herself a ridiculous amount of butt). She stared at me for a while (I must admit, she's a little big) and she walked away and sat with her friends and she asked them if she was fat and started asking if she should start losing weight and going to the gym and oh God the whole ordeal was awful.
I think one thing that might be possible about this whole topic is not that certain types do understand sarcasm and others don't, but rather that different types communicate sarcasm in different ways.
It's possible that N types understand each others' sarcasm more than S types would, but that S types perfectly understand sarcasm between themselves.
In regards to ISFJs vs. INTPs, I think a lot of it is that for ISFJs things kinds of things are so much based on trust and relationships. My best friend is an INTP, and I've been friends with him for about 20 years. I understand his humor and am not hurt by his jokes about me/his picking on me because I understand that he deeply values me as a friend even if he doesn't communicate that directly. I understand that he likes to make jokes just to make things interesting and just to get us laughing.
I also think being around him for so long makes it easier for me to recognize when he's joking and when he's not...I recognize his mannerisms and I know the kinds of things that make him laugh.
But, I imagine that if someone else, INTP or not, said some of the same things that he did, I might not understand that they were joking or I might be hurt by it.
So I think different factors play into it with each type...each type's understanding of sarcasm can just be different. I don't know if it's a matter of some types "getting it better" than other types.