People might be aware that submarines predates 20,000 Leagues Under the Sea (the first submarine named Nautilus dates to 1800), though a surprising number aren't. In general, mechanical things seems to be the things people are most likely to believe are that old.
But when he described a fax-like thing in his story Paris in the Twentieth Century, people tend to be sure it was a fantastic prediction.
Only the book was written in 1863, and the first commercial fax-like service - the Pantelegraph - went into commercial operation between Paris and Lyon in 1863, and the invention happened many years before. Becquerel, Rossini and Napoleon III all were involved in demonstrations.
Outside of Jules Verne, my other favourite tech that people think is more modern than it is, is video telephony. But when you think about it, the crudest version you can build of video telephony is "just" a pair of TV cameras and a pair of screens. Hence the video-telephony network set up in Germany in the 1930's (but people are often surprised at how old "real" video-telephony is too; people are even surprised to hear about CU-SeeMe these days; then again my son is surprised I had electricity when I grew up given I predate Youtube)
I think a lot of it is because we think in terms of the modern conception of what something is. But as you can see if you look up the Pantelegraph, you can do "fax-like" fairly easy: The early variants tended be variants over the theme of breaking a current (like the Pantelegraph) or using magnetic ink with a simple scanning device that caused the resulting current to directly drive the rendering on the other side. And video-telephony a la Skype or Zoom is complex, but video-telephony a la the most basic camera and screen is simple once you have the cameras and screens.
We see the same with photographs. If you look at modern cameras, they're of course complex and it's easy to imagine them as modern (imagine what it will seem like to kids growing up without having seen mechanical cameras...) but look back at the processes and the simplest cameras and simplest photographic processes are almost literally a science experiment for kids now that we know how to do it, and the genius lay in figuring out the concept.