Funny enough I'm an A̶I̶ML researcher and started in HPC (High Performance Computing).
> if I’m a C++ programmer ... should I know how the CPU registers are called?
Probably.
Especially with "low level"[0] languages knowing some basics about CPU operations goes a long way. You can definitely get away without knowing these things but this knowledge will reap rewards. This is true for a lot of system based information too. You should definitely know about things like SIMD, MIMD, etc because if you're writing anything in C/C++ these days it should be because you care a lot about performance. There's a lot of stuff that should be parallelized that isn't. Even stuff that could be trivially parallelized with OpenMP.
> what should an AI researcher working always in Jupyter know?
Depends on what they're researching. But I do wish a lot more knew some OS basics. I see lots of things in papers where they're like "we got 10x" performance on some speed measurement but didn't actually measure it correctly (e.g. you can't use time.time and be accurate because there's lots of asynchronous operations). There's lots of easy pitfalls here that are not at all obvious and will look like they are working correctly. There's things about GPUs that should be known. Things about math and statistics. Things about networking. But this is a broad field so there are of course lots of answers here. I'd at least say anyone working on AI should read at least some text on cognitive science and neuroscience because that's a super common pitfall too.
I think it is easy to not recognize that information is helpful until after you have that information. So it becomes easy to put off as not important. You are right that it is really difficult to balance everything though but I'm not convinced this is the problem with those in that category of programmers. There's quite a number of people who insist that they "don't need to" learn things or insist certain knowledge isn't useful based on their "success."
IMO the key point is that you should always be improving. Easier said than done, but it should be the goal. At worst, I think we should push back on anyone insisting that we shouldn't be (I do not think you're suggesting this).
[0] Quotes because depends who you talk to. C++ historically was considered a high level language but then what is Python, Lua, etc?