Oh, I wouldn't be surprised at all. I've worked in IT support for a couple of decades now, and I know exactly how that goes.
There are two things about that though. First, it's a bit of a fringe case. You have to consider the question, "How many people of that sort are in my target audience?" If the answer is "a lot", then you should think about writing documentation for them specifically, and find a way separate it out from other documentation for those who are more comfortable using a computer. Otherwise, people who know what they're doing are going to be frustrated searching through 100 pages of inane instructions to find actual information.
Second, people like that often also won't read the documentation. If they do, they won't understand it, or else won't feel confident that they understand it. At a certain point, you have to either provide those people with IT support personnel that they can call (your older relative has you). At the very least, you need to provide them with simple step-by-step instructions that never vary, where they don't even need to understand what they're doing. Like "In order to do [x], press the power button on your computer to turn it on (it's located in the top-right-hand corner of the box under your desk). It will flash some things on the screen for a while. Wait for it to ask for a password, and then type 'hunter2'. Wait 2 minutes. Then find the blue "E" on your screen, with "Internet Explorer" written under it. It will be the third little picture on the screen, all the way to the left..."
I've had to write instructions like that before, and some people need it to be that simple. But obviously a web application vendor can't take responsibility for that level of instruction. Even something like Dropbox, which is designed to be extremely simple, has to assume some level of competency.